BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to railroads generally, and more particularly to a method and system for identifying problems with train tracks. 
     2. Discussion of the Background 
     Track circuits of various types have been used for many years in the railroad industry to determine whether sections or blocks of train track are safe for transit. These track circuits determine such things as whether there is a train in a section of track, whether there is a broken rail in a section of track, whether there has been an avalanche or whether snow or other debris is on the section of track, and whether the section of track is properly aligned with a bridge (with moveable and/or permanent spans). These and other such track circuits will be referred to herein as “track integrity circuits” or simply “track circuits.” 
     Some known circuits combine the functions of detecting broken rails and detecting trains in a section of track. In their simplest form, these circuits involve applying a voltage across an electrically discontinuous section of rail at one end and measuring the voltage at the other end. If a train is present between the point at which the voltage is applied and the point at which the measuring device is located, the wheels and axle of the train will short the two rails and the voltage at the other end of the track will not be detected. Alternatively, if there is a break in one of the rails between the point at which the voltage is applied and the point at which the voltage measuring device is located, the voltage won&#39;t be detected. Thus, if the voltage cannot be detected, there is either a break in the rail or the track is occupied by another train. In either event, it is not safe for a train to enter the section of track monitored by the track circuit. 
     Many variations of such circuits have been proposed. Examples of such circuits can be found in U.S. Pat. Nos. 6,102,340; 5,743,495; 5,470,034; 5,145,131; 4,886,226; 4,728,063; and 4,306,694. These circuits vary in that some use A.C. signals while other employ D.C. signals. Additionally, some of these circuits employ radio links between the portions of the circuit which apply the signal to the rails and the portions of the circuit that detect the signals. There are yet other differences in these circuits. These differences are not important within the context of the present invention and any of these circuits may be used in connection with the invention. 
     In traditional systems, the track circuit was connected to a wayside color signal to indicate the status of the track to approaching trains and the track circuit operated continuously or periodically regardless of whether any train was approaching the section of track monitored by the track circuit. There are two major problems with such systems. First, the operation of the track circuit in the absence of an oncoming train wasted power. This limited the use of such systems to locations near a source of power. Second, the use of wayside signals was not failsafe in that it required the conductor/engineer to observe the signal and stop the train when the signals indicated that there was a problem such as a train on the track or a broken rail. Because human beings are not perfect, signals were sometimes missed and accidents resulted. 
     Some known systems solve the first problem by activating the track detection circuit only when a train is approaching. For example, U.S. Pat. No. 4,886,226 describes activating a broken rail circuit only when an approaching train triggers a “feed” positioned before the section of track monitored by the track circuit. While this solution does conserve power and allow the broken rail detection circuit to be used with a solar cell or battery power source, it has the disadvantage of high maintenance costs associated with the “feed”. Another prior art system described in U.S. Pat. No. 4,728,063 requires a dispatcher to monitor a location of a train and activate a broken rail detection circuit by radio when the train nears the end of the block. The status of the track as reported by the broken rail detection circuit is then transmitted back to the dispatcher, who in turn passes it along by radio to the train. This system is inefficient in that it places an increased processing load on the dispatcher, as the dispatcher is forced to receive and send such messages each time each train reaches a new track circuit. It is also problematic when communications between the dispatcher and the broken rail detection circuit become interrupted. 
     Approach lit signaling is also know in the art. In those system, the signal lights are only lit when a train approaches the signal. However, in the systems known to the inventors, the track integrity circuit remains on even when the signal lights are out (the main reason the signal lights are turned off is to make the signal lights less attractive to vandals). Furthermore, the track integrity circuits in these systems conserve relatively large amounts of power. These systems are therefore not suitable for use with solar and/or battery power. 
     What is needed is a method and system for activating track circuits in an economical manner that allows such circuits to be used in a way that minimizes power consumption while avoiding undue burden on a dispatcher or other control authority. 
     SUMMARY OF THE INVENTION 
     The present invention meets the aforementioned need to a great extent by providing a computerized train control system in which a control module determines a position of a train using a positioning system such as a global positioning system (GPS) and consults a database to determine when the train is approaching a portion of track monitored by a track circuit. When the train is approaching a track circuit, but while the train is still far enough away from the track circuit that the train can be stopped before reaching the portion of track monitored by the track circuit, the train transmits an interrogation message to a transceiver associated with the track circuit. In preferred embodiments, the message is transmitted wirelessly to the track circuit. Other transmission methods are also possible, including transmitting an interrogation message to a transceiver associated with the track circuit via one or both of the rails. When the track circuit receives the interrogation message, a test is initiated. The results of the test are transmitted back to the train, which then takes some form of corrective action if the track circuit indicates a problem. 
     In some embodiments, the train will come to a complete stop before reaching the portion of the track monitored by the track circuit when a problem is indicated. In other embodiments, if the engineer/conductor acknowledges a message warning of the problem and slows the train to a safe speed, the system will allow the train to proceed at the safe speed while the engineer/conductor visually determines whether it is safe to continue. In such embodiments, the system will stop the train if the engineer/conductor fails to acknowledge the warning message or fails to slow the train to a safe speed. Preferably, the safe speed is determined on the basis of the weight of the train as well as other characteristics (e.g., the grade of the track, the distribution of the weight on the train, etc.) that affect braking distance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant features and advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a logical block diagram of a train control system according to one embodiment of the invention. 
         FIG. 2  is a flow chart of processing performed by the train control system of  FIG. 1  in one embodiment of the invention. 
         FIGS. 3   a  and  3   b  are a flow chart of processing performed by the train control system of  FIG. 1  in a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be discussed with reference to preferred embodiments of train control systems. Specific details, such as specific track circuits and signals, are set forth in order to provide a thorough understanding of the present invention. The preferred embodiments discussed herein should not be understood to limit the invention. Furthermore, for ease of understanding, certain method steps are delineated as separate steps; however, these steps should not be construed as necessarily distinct nor order dependent in their performance. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views,  FIG. 1  is a logical block diagram of a train control system  100  according to an embodiment of the present invention. The train control system includes a train unit  105  and a plurality of pairs of track circuits  180  and transceivers  190  that monitor various sections of track  185 . These track circuit  180 /transceiver  190  pairs may be placed only at certain locations on the track  185  (e.g., only near mountainsides when the track circuits  185  are of the form of avalanche detection circuits), or may be positioned such that the entire length of track is monitored. It should also be noted that the track circuit  180  is not necessarily connected to the track rails themselves as is shown in FIG.  1 . For example, avalanche detection circuits are typically connected to slide fences rather than to the track itself. In this case, the circuits detect breaks in the-slide fences, which indicate that debris has broken through the fence and, potentially, onto the track. 
     The train unit  105  includes a control module  110 , which typically, but not necessarily, includes a microprocessor. The control module  110  is responsible for controlling the other components of the system. 
     A positioning system  120  is connected to the control module  110 . The positioning system supplies the position (and, in some cases, the speed) of the train to the control module  110 . The positioning can be of any type, including a global positioning system (GPS), a differential GPS, an inertial navigation system (INS), or a Loran system. Such positioning systems are well known in the art and will not be discussed in further detail herein. (As used herein, the term “positioning system” refers to the portion of a positioning system that is commonly located on a mobile vehicle, which may or may not comprise the entire system. Thus, for example, in connection with a global positioning system, the term “positioning system” as used herein refers to a GPS receiver and does not include the satellites that transmit information to the GPS receiver.) 
     A map database  130  is also connected to the control module  110 . The map database  130  preferably comprises a non-volatile memory such as a hard disk, flash memory, CD-ROM or other storage device, on which map data is stored. Other types of memory, including volatile memory, may also be used. The map data preferably includes positions of all track circuits in the railway. The map data preferably also includes information concerning the direction and grade of the track in the railway. By using train position information obtained from the positioning system  120  as an index into the map database  140 , the control module  110  can determine its position relative to track circuits. 
     When the control module  110  determines that the train is approaching a track circuit  180  (which includes a transceiver  190 ) that monitors a section of track  185  and is within range for conducting communications, it interrogates the device  180  through transceiver  150 . The transceiver  150  can be configured for any type of communication, including communicating through rails and wireless communication. In addition to communicating with track circuit transceivers  190 , the transceiver  150  may communicate with transceivers connected to other devices such as switches and grade crossing gates, and may also communicate with a dispatcher (not shown in  FIG. 1 ) from whom route information and track warrants and authorities are received. In other embodiments, separate communications devices are used for wayside device communication and communication with a dispatcher. 
     Also connected to the control module  110  is a brake interface  160 . The brake interface  160  monitors the train brakes and reports this information to the control module  110 , and also allows the control module  110  to activate and control the brakes to stop or slow the train when necessary. 
     A warning device  170  is also connected to the control module  110 . The warning device  170  is used to warn the conductor/engineer that a malfunction has been detected. The warning device  170  may also be used to allow the engineer/conductor to acknowledge the warning. In some embodiments, the warning device  170  is in the form of a button on an operator display such as the display illustrated in co-pending U.S. application Ser. No. 10/186,426, entitled, “Train Control System and Method of Controlling a Train or Trains” filed Jul. 2, 2002, the contents of which are hereby incorporated by reference herein. In other embodiments, the warning device  170  may be a stand-alone button that illuminates when a malfunction is detected. In yet other embodiments (e.g., those in which no acknowledgment of a warning is required), the warning device  170  may comprise or consist of a horn or other device capable of providing an audible warning. 
       FIG. 2  is a flowchart  200  illustrating operation of the control module  110  in connection with a track circuit  180  in one embodiment of the invention. In this embodiment, which is particularly well suited for use with track circuits such as broken rail detection circuits and avalanche detection circuits, the train will be preferably be brought to a complete halt, either by the operator or automatically by the control module  110  if the operator fails to take action, before reaching the section of track monitored by the track circuit. Forcing the train to come to a complete stop forces an operator to make a positive decision to move the train forward through the section of track indicated as bad, thereby dramatically decreasing the chances that the operator will miss the warning provided by the track circuit. In some embodiments of the invention, permission from the dispatcher is required before the control module  110  will allow the train to move again. 
     The control module  110  begins the process by obtaining the locations of nearby track circuits  180  from the map database  130  at step  210 . The control module  110  then determines the train&#39;s current position from information provided by the positioning system  120  at step  212 . If no track circuit  180  is within a threshold distance, steps  210  et seq. are repeated. If a track circuit  180  is within a threshold distance at step  214 , the transceiver  190  associated with the track circuit  180  is interrogated at step  216 . 
     In some embodiments, this threshold distance is a predetermined distance based upon the communication ranges of the transceiver  150  on the train and the transceiver  190  connected to the track circuit  180 . In other embodiments, the threshold distance is equal to a distance required to stop the train under a worst-case assumption (i.e., an assumption that a train having the greatest possible weight is traveling at a maximum allowable or possible speed in a downhill direction on a portion of track with the steepest grade in the system) plus an offset to allow the track circuit to perform the track test and respond to the interrogation. In yet other embodiments, the threshold is dynamically determined based on the actual speed and weight of the train and the grade of the track between the train and the track circuit such that there is sufficient time for the track circuit  180  to test the track  185  and report the results in response to the interrogation. In other embodiments, the calculation may take into account the distribution of weight in the train as this will effect the required stopping distance as discussed in the aforementioned co-pending U.S. patent application. 
     In some embodiments, the interrogation includes an identification number associated with the track circuit  180 . This identification number is obtained from the map database  130 . Only the track circuit corresponding to the identification number will respond to the interrogation. This avoids contention between multiple devices (track circuits or other devices—e.g., switches, crossing gates, etc.) attempting to respond to the interrogation on the same frequency. Thus, by assigning unique device numbers to track circuits and other devices, all devices can share the same frequency. 
     If the track circuit  180  fails to respond at step  218 , or reports a problem with the track at step  220 , the control module  110  warns the conductor/engineer of the problem via the warning device  170  at step  224 . The control module  110  then determines whether the brakes have been activated at step  226  by communicating with the brake interface  160  directly and/or by obtaining speed information from the positioning system  120 . Preferably, the control module  110  calculates the braking force necessary to stop the train prior to reaching the section of track monitored by the track circuit  180  taking into account the speed and weight of the train, the distribution of the weight on the train, the grade of the track, and the characteristics of the braking system itself. If the operator has not activated the brakes in a manner sufficient to stop the train in time at step  226 , the control module  110  automatically activates the brakes to stop the train at step  228 . 
     If the track circuit  180  responds to the interrogation at step  218  and reports that the track  185  is intact at step  220 , then the control module  110  returns to step  210  to repeat the process. Returning to step  210  will result in interrogating the track circuit  180  device multiple times as the train approaches. This is desirable for safety purposes because it will detect any problems that occur after the initial interrogation (e.g., a vandal dislodging a rail) from causing and accident. 
     Whether or not the interrogation of step  218  includes the device&#39;s identification number, it is preferable for the device&#39;s response to include its identification number as this allows for greater assurance that a response from some other source has not been mistaken as a response from the track circuit  180  of interest. 
       FIGS. 3   a  and  3   b  together form a flowchart  300  illustrating operation of the control unit  10  in connection with configurable devices  180  according to a second embodiment of the invention. This embodiment allows a train to proceed through a section of track at a reduced speed such that the train can be stopped if the operator visually determines that there is a problem with the track (e.g., a broken rail or another train on the tracks) rather than forcing the train to come to a complete halt. This is done because track circuits sometimes give a false indication of a problem. Steps  310 - 320  of the flowchart  300  are similar to steps  210 - 220  of the flowchart  200  of  FIG. 2 ; therefore, the detailed discussion of these steps will not be repeated. 
     If a track circuit  180  does not respond at step  318  or reports a problem with the track  185  at step  320  after being interrogated at step  316 , the control module  110  activates the warning device  170  at step  330 . When the warning device  170  is activated, the operator/engineer is given a period of time in which to acknowledge the warning and slow the train to a speed that is slow enough to allow the operator to stop the train before reaching a problem (e.g., a broken rail or another train on the track) that the operator detects visually. This period of time may be predetermined based on a worst-case assumption of required distance to stop the train if the operator doesn&#39;t acknowledge the problem and slow the train to the safe speed, or may be determined dynamically based on factors such as the current speed of the train, the braking characteristics of the brakes on the train, the weight of the train, the distribution of weight on the train, and/or the grade of the track as determined from the map database  130  using the train position from the positioning system  120 , as well as other factors that affect the required stopping distance/time. 
     If the operator acknowledges the warning at step  332  and reduces the speed of the train to the safe speed at step  334  within the allowable time period, the control module  110  monitors the train&#39;s speed such that the reduced speed is maintained at step  336  until the train has passed through the section of track monitored by the track circuit  180  at step  338 . 
     If the conductor/engineer fails to acknowledge the warning at step  332  or fails to reduce the train&#39;s speed to the safe speed at step  334  within the allowed time period, the control module  110  commands the brake interface to stop the train at step  342 . The control module  110  then notifies the dispatcher of the stopped train at step  344 . 
     One advantage of those embodiments of the invention in which a configurable device is interrogated as the train approaches is that such devices are not required to transmit information when trains are not in the area. This saves power as compared to those systems in which wayside devices continuously or periodically transmit information regardless of whether a train is close enough to receive such information. 
     As discussed above, preferred embodiments of the invention include an identification number in the interrogation messages sent to transponders  190  associated with track circuits  180 . However, it is also possible to transmit interrogation messages without identification numbers, in which case each transporter that receives the interrogation will respond and include an identification number in its response. In either case, this allows all transponders to share the same frequency, which reduces complexity and cost. 
     In the embodiments discussed above, the control module  110  is located on the train. It should also be noted that some or all of the functions performed by the control module  110  could be performed by a remotely located processing unit such as processing unit located at a central dispatcher. In such embodiments, information from devices on the train (e.g., the brake interface  160 ) is communicated to the remotely located processing unit via the transceiver  150 . 
     One particularly important advantage of the invention is that it facilitates use of track circuits in remote areas. That is, because an approaching train transmits an interrogation message, the track detection circuit need only be “on” when the train approaches and may be in a low-power standby or off state with the transceiver in a low power “listening state” at other times when no train is nearby. This in turn facilitates the use of solar cells as a power source for these track circuit/transponder combinations. Furthermore, no high-maintenance mechanical device is required to detect the presence of the train. An important consequence of this is that the invention provides the ability to include broken rail protection in dark territory in which no power source is available at low cost. 
     Another important aspect of the invention is its failsafe nature. Because the control unit  110  ensures that corrective action is taken if the track circuit  180  does not respond to an interrogation, there is no danger if the track circuit  180  and/or the track circuit transceiver  190  fails to respond, thereby making the system failsafe. This also eliminates the need to perform preventive maintenance. Additionally, no signal lights are necessary, which eliminates a failure mode. Maintenance costs are dramatically reduced as a consequence of these two aspects. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
A train control system includes a control module that determines a position of a train using a positioning system and consults a database to determine when the train is approaching a portion of track monitored by a track circuit. When the train is near a track circuit, but while the train is still far enough away from the track circuit such that the train can be stopped before reaching the portion of track monitored by the track circuit, the train transmits an interrogation message to a transceiver associated with the track circuit. When the track circuit receives the interrogation message, a test is initiated. The test results are transmitted back to the train. The train takes corrective action if the track circuit fails to respond or indicates a problem.