BACKGROUND OF THE INVENTION 
     The present invention relates to improvements in railroad tank cars, and in particular to such a car with reduced car weight, and having integrated and unified load-bearing structures so that dynamic train loads and static loads are shared between the tank itself and the underframe portion of the structure of the tank car. 
     Many railway tank cars have completely separate underframes with tank support structures mounted on the underframes and tank bodies attached to the support structures. While such cars have ample strength it is desired to provide tank cars of lighter weight, since a car whose tare weight is less can carry greater weights of cargo and thus generate greater amounts of revenue using a given amount of fuel. 
     A type of tank car structure used extensively in North America has a stub sill supported by a wheeled truck attached to each end of a tank. The structure of the tank supports itself between the stub sills, carrying not only the static and dynamic loads resulting from the weight and movement of liquid cargo contained in the tank, but also dynamic loads resulting from operation of the car as part of a train. In such cars forces are concentrated in and carried through the locations where the stub sills are attached to the tank. As a result, such cars all too frequently are subject to structural failure during train operation. 
     A type of tank car structure used widely on European railroads utilizes a relatively heavy underframe including a substantial longitudinal frame structure upon which are carried saddles to support a tank. Cargo loads are transferred to the underframe of such a car by the saddle structures alone, and a relatively massive center sill structure, separate from the tank, is utilized to carry most dynamic train loads. Such cars are significantly heavier than is desired, for a given liquid cargo weight capacity. 
     Another result of such a structural design is that the end portions of the underframe of such a car have to be of relatively heavy construction in order to permit the car when loaded to be supported on jacks located at the corners of the underframe, since there is a relatively long lever arm between the corners of the underframe and the saddle attachment locations, where the weight of the tank and included cargo is transferred to the underframe. 
     A further deficiency of such a railroad tank car design is that the structures of the tanks and the underframes of the cars do not cooperate with each other to carry dynamic loads during operation of such cars as part of a train. 
     What is needed, then, is an improved railroad tank car of reliably durable construction but lighter in weight than previously known cars. Additionally, it is desired for such a car to be able to be assembled with a minimum of special skill requirements and a minimum of requirements for formed metal parts included in such a car&#39;s structure. 
     SUMMARY OF THE INVENTION 
     The present invention provides a railroad tank car structure which answers the aforementioned need for a tank car of ample strength and durability combined with lighter car weight and simple construction, by providing a car in which the tank shell and underframe components are better integrated into a unified, strong, and light structure than has previously been accomplished. 
     In one preferred embodiment of the present invention a railroad tank car includes a pair of transversely extending saddle bolsters interconnected by a center sill structure, and a stub sill extends outward longitudinally from each saddle bolster. A tank is closely supported by the saddles of the saddle bolsters. The tank also rests atop and is fastened, preferably welded, to the center sill and to each stub sill, so that the stub sills, saddles, tank, and center sill are a unified structure and cooperate closely to carry the static loads imposed by the weight of cargo carried in the tank and the dynamic loads that result from operation of a train including such a car and its cargo. 
     In one embodiment of the present invention each saddle is connected with the tank over a significant portion of the outside of the tank and provides support for the tank against atomospheric “vacuum pressure” in the case of improper venting of the tank during discharge. 
     In one preferred embodiment of the present invention the interconnection of the tank with the stub sills, saddle bolsters, and center sill includes the use of doubler plates that carry and distribute forces among the various portions of the tank car through structure of great enough size that critical force concentrations are avoided, while the car&#39;s weight is reduced. 
     In a preferred embodiment of the invention doubler plates are designed to function as linear stiffeners in transition zones between joints. 
     In one embodiment of the invention stub sills, saddle bolsters, and a center sill are constructed primarily by welding flat steel plate parts so that construction of such a car is uncomplicated. 
     In one embodiment of the present invention the stub sills include torsion box structures interconnecting the bottom plate and top shear plate of the stub sill and providing ample stiffness in the stub sill. 
     In one embodiment of the invention a portion of the tank located longitudinally outward from the saddle bolsters rests atop and is fastened to the top shear plate of the stub sill. 
     In a preferred embodiment of the invention, each stub sill includes a coupler tube capable of accepting either European hook-and-link couplers or North American type automatic knuckle couplers and associated cushioning devices. 
     The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a railroad tank car embodying the present invention. 
     FIG. 2 is an isometric view of the car shown in FIG. 1, taken from above and to one side of an end of the car. 
     FIG. 3 is an end elevational view of the body of the car shown in FIG.  1 . 
     FIG. 4 is a sectional view, at an enlarged scale, of a portion of the car shown in FIG. 1, taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is a top plan view of the car shown in FIG.  1 . 
     FIG. 6 is a section view of the body of the car shown in FIG. 1, taken along line  6 — 6 , at an enlarged scale. 
     FIG. 7 is a top plan view, at an enlarged scale, of the transversely extending saddle bolster the stub sill structure, and a portion of the center sill structure of the car shown in FIG. 1, with a portion of the tank shown in broken line. 
     FIG. 8 is an isometric view of the structures shown in FIG. 7, with a portion of the tank shown in broken line, as seen from near one end of the car body. 
     FIG. 9 is an isometric view of the saddle and sill structures shown in FIG. 8 with most of the top shear plate of the stub sill removed to provide a better view of the interior structure of the saddle bolster and stub sill. 
     FIG. 10 is an isometric view of a portion of the railroad tank car body shown in FIG. 1, taken from one side of and below a middle part of the car body, looking longitudinally outward toward an end of the car body. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings which form a part of the disclosure herein, a tank car  16  which is one preferred embodiment of the present invention includes a substantially unitized body  18  including an underframe portion  20  and a tank  22 , supported on a pair of wheeled trucks  24 , as may be seen in FIGS. 1 and 2. 
     The tank  22  includes a pair of opposite ends  26 , a top  28  and a bottom  30 , seen in FIG.  3 . The tank has two generally cylindrical halves joined together at the midpoint of the length of the car, and defines generally a longitudinal central axis  32 . The two longitudinal halves of the tank  22  are inclined downward slightly toward the mid-portion of the car body  18 , at an angle of, for example, about 1° from horizontal, to provide for drainage. A pair of ladders  34  extend upwardly on opposite sides of the tank  22  toward its top  28 . 
     The underframe  20  includes a pair of saddle bolsters  36  and  38 , each located centrally above a conventional center bearing for the respective one of the trucks  24 . Each of the saddle bolsters  36  and  38  includes a transverse lower portion  40  including a location for a foundation on each side of the car  16  for a respective side bearing for the respective truck  24 . Each saddle bolster  36  and  38  extends upwardly, and includes a concave saddle portion  42  that fits matingly against and is welded to a lower portion of the tank  22 . 
     Extending longitudinally of the car between the saddle bolsters  36  and  38  is a center sill  44 . The ends of the center sill  44  are welded to the saddle bolsters  36  and  38 , and the center sill  44  also is welded to the bottom  30  of the tank  22  along its entire span between the saddle bolsters  36  and  38 . 
     Attached to the longitudinally outward side of each of the saddle bolsters  36  and  38  is a respective stub sill  50  whose top shear plate  52  rests against and is welded to a portion of the bottom  30  of the tank  22 , adjacent a respective one of the ends  26 , so that the tank  22  is connected closely with and supported by the saddle bolsters  36 , the center sill  44 , and the stub sills  50 . 
     At an outer end of each stub sill  50  are a pair of buffer foundations  54  and conventional buffers  56 , as well as a centrally located coupler housing or tube  58 , with a conventional striker plate  60 . The coupler tube  50  is thus configured to accept either the conventional European hook-and-link type coupler as shown at  62  in FIG. 1 or an automatic coupler (not shown) including a cushioning device. 
     As shown in FIGS. 2,  3 , and  4 , the saddles  42  extend upward in intimate contact with the outside of the tank  22  on each side of the car body  18 , subtending an angle  66  about the central axis  32 , preferably in the range of 110-140°, thus giving lateral and vertical support to unify the tank  22  with the underframe  20 , and also providing structural support for the skin or shell  68  of the tank  22 . 
     The saddle bolsters  36  and  38  are substantially identical with each other, each including a pair of similar transversely and vertically extending flat plates, an inner plate  70  and a longitudinally outer plate  72  which may be of steel with a thickness of 4.5 mm. The plates  70  and  72  are spaced apart from each other along the length of the car body  18  by a distance  74 , center-to-center, of 340 mm. Because of relevant regulations in some localities, and for adequate strength, the distance  74  should be in the range of 300 to 360 mm. A bottom closing plate  76 , a side closing plate  78 , and a saddle doubler plate  80  extend along and are welded to respective margins of the inner and outer plates  70  and  72  of the saddle bolsters  36  and  38 . The bottom plate  76  preferably has a thickness of 8 mm, while the side closing plate  78  may be 4-5 mm thick and the doubler plate  80  may be 4-6 mm thick. A U-shaped closer piece  82  of similar material interconnects the upper margins of the inner and outer plates  70  and  72  with each other and with the side closing plate  78  and the doubler plate  80  at each upper end of the saddle portion  42 . 
     The saddle doubler plate  80  extends longitudinally of the car beyond the inner and outer plates  70  and  72  and also extends upward along the respective side of the tank  22  beyond the inner and outer plates  70  and  72  by an overlap distance  84  of at least 15 mm, and preferably about 50-75 mm, in order to reinforce the shell  68  of the tank  22  and provide an amply large area of interconnection between the doubler plates  80  and the shell  68 , and the doubler plates  80  are welded to the shell  68  around their outer margins. 
     Extending from each saddle bolster  36  or  38  toward the center portion of the car  16 , the length of the center sill  44  includes a pair of parallel upright webs  90  each welded to the inner plate  70  of each saddle bolster and extending longitudinally of the car body  18 . A narrow transversely extending bottom flange member  92  extends along and is welded to the bottom margin of each of the webs  90  and may be of steel plate about 18 mm thick and about 100 mm wide, for example. Cross tie members  94  may be of angle stock and are welded to the bottom flanges  92 , extending between and tying together the webs  90 , parallel with the width  95  of the center sill  44 , as shown in FIGS. 1,  3 ,  5  and  10 . 
     A respective portion of a tank bottom doubler plate  96  is welded to an upper margin of each web  90 . The tank bottom doubler plate  96  also extends along and is welded to the bottom  30  of the tank  22  and is therefore formed into a partial cylinder to fit matingly against the outer surface of the bottom  30  of the tank  22 . The tank doubler plate  96  thus acts as a part of the center sill  44  and also interconnects the center sill  44  with the shell  68  over a significant area of the bottom  30 . As may be seen in FIGS. 1 and 6, the webs  90  of the center sill are of shallower depth in the longitudinally central portion of the car body  18  than closely adjacent the saddle bolsters  36  and  38 . 
     The ladders  34  extend from and include portions of a pair of rings  100  each extending around the shell  68  of the tank  22  near mid-length of the tank  22 . The rings  100  are preferably of steel plate, each having a thickness of about 6 mm and a radial depth or width  102  of about 75 mm. The two rings  100  are spaced apart from each other longitudinally of the car body  18  by a distance  103  of about 40 cm, for example, appropriate to act as the rails of the ladder  34 , and are interconnected with each other by the several rungs  104  of the ladders  34 . Each of the rings  100  is welded to the shell  68  of the tank  22 , so that the pair of rings  100  act cooperatively to provide radial support and stiffening for the shell  68  of the tank  22 , to help it to withstand atmospheric pressure in case of insufficient venting during emptying of cargo from the tank  22 . 
     Referring now also to FIGS. 7 and 8, it will be seen that a full-width portion  106  of the tank bottom doubler plate  96  extends across and is welded to the center part of the upper margins of the vertical transverse plates  70  and  72  of the saddle  42 , interconnecting the closer or bottom ends of the saddle doubler plates  80  with each other, and interconnecting the upper margins of the plates  70  and  72 . A narrower end portion  108  of the doubler plate  96  extends longitudinally outward away from the saddle portion  42  of the saddle bolsters and is closely in contact with and welded along its margins to the outside of the shell  68  of the tank  22 . The doubler plate  96  thus carries forces between the shell  68  of the tank  22 , the inner and outer plates  70  of the saddle, and the webs  90  of the center sill. A large opening  110  is defined in the tank bottom doubler plate  96  between the parallel webs  90  of the center sill, at a distance  112  longitudinally inward from the inner plate  70 , leaving a solid portion of the tank bottom doubler plate  96  adjacent the inner plate  70  large enough to spread the forces transmitted between the shell  68  and the saddle bolster  36  or  38  over a safely large area of the tank shell  68 . 
     Referring to FIGS. 2,  3 ,  4 ,  5 ,  7 , and  8 , each stub sill  50  is generally trapezoidal in plan shape, having a maximum width  114  greater than the width  95  of the center sill and extending longitudinally outward from the outer plate  72  of the adjacent saddle bolster  36  or  38 . The stub sill  50  includes a pair of upright end plates  116  located on opposite sides of the coupler tube  58  and extending transversely, supporting the buffer foundations  54 . A pair of upstanding sideplates  118  extend generally longitudinally and diverge obliquely apart from the outer plate  72  of the saddle bolster  36  or  38  to a corner of the stub sill  50  adjacent a respective one of the end plates  116 . The top shear plate  52  extends along and is welded to an upper margin of each of the sideplates  118  and to an upper margin of each of the end plates  116 . A bottom plate  120  extends generally horizontally along respective bottom margins of the sideplates  118  and the end plates  116  and is securely welded to the sideplates  118  and end plates  116 , as well as to the striker plate  60 . The bottom plate  120  butts into and is welded to the margin of the bottom plate  76 , which serves as a foundation for a conventional center bearing (not shown) to support the car body  18  on the trucks  24 . 
     The sideplates  118  are tapered in height from a greatest height adjacent the outer plate  72  to a smaller height adjacent the end plates  116  at the longitudinally outer end of the stub sill  50 . The top shear plate  52  is thus inclined upward at a shallow angle from the upper margin of the end plates  116  toward the bottom  30  of the tank  22  adjacent the end  26 . The sideplates  118  may be of steel plate having a thickness of 4-6 mm. The bottom plate  120  may be of steel with a thickness of 6 mm. 
     The top shear plate  52  has an inboard margin  122  shaped to correspond matingly with the surface of the bottom  30  of the tank  22  and the bottom portion of its end  26 . A narrow doubler plate  124  extends along, and is welded to, a portion of the upper margin of each side plate  118 , the inner margin  122  of the top shear plate  52 , and respective portions of the upper margins of the sideplates  118 , as well as being welded to the outside of the shell  68  of the tank  22 , so that the tank  22  is securely fastened to the stub sill  50 . The doubler plate  124  may have a thickness of 4.5 mm and a width  126  of 60 mm, to spread the forces transferred between the shell  68  of the tank  22 , the top shear plate  52 , and the sideplates  118  over an area safely large enough to avoid dangerous concentrations of stress in the attachment of the tank  22  to the underframe  20 . Thus, the structures of the stub sill  50 , the tank  22 , and the saddle bolster  36  or  38  are integrated by interconnection with each other over areas sufficient to avoid excessive concentrations of stresses. 
     As may be seen in FIG. 9, in which most of the top shear plate  52  is removed from the stub sill  50 , longitudinal webs  130  located between the inner and outer plates  70  and  72  are aligned with and extend between the adjacent margins of the webs  90  of the center sill  44  and the inboard margins  132  of the sideplates  118 . Additional strengthening webs  134  may be used to interconnect the inner and outer plates  70  and  72  of the saddle bolster  36  or  38  at locations spaced laterally outward from the center sill  44  to support the side bearings between the trucks  24  and the car body  18 . 
     The bottom plate  120  defines a pair of laterally opposite openings  136  and a U-shaped central opening  138  beneath the coupler housing tube  58 . Additionally, both the top shear plate  52  and the bottom plate  120  define pairs of correspondingly shaped openings  140  and  142 , respectively, which are located above each other and are generally similar. Because the top shear plate  52  is inclined at a small angle with respect to the bottom plate  120 , the openings  140  and  142  differ slightly in shape. A respective stiffener sheet  144  which may be an assembly of several flat portions interconnected by arcuately formed partial cylinder portions is connected to both said top shear plate  52  and said bottom plate  120  near, or preferably along, the margins of each set of the openings  140  and  142  and forms a tubular interconnection between the top shear plate  52  and the bottom plate  120 . The respective tubular stiffener sheet  144  is welded to both the top shear plate  52  and the bottom plate  120  about the entire periphery of each of the openings  140  and  142  so that each stiffener sheet  144  forms a torsionally stiff box interconnecting the top shear plate  52  and the bottom plate  120 . The stiffener sheets  144  may, for example, be of steel plate 4.5 mm in thickness. 
     It will be understood further that the stiffener sheet  144  could form a tube larger than each or one of the openings  140  and  142 , if desired, although one object of the opening  140  and  142  is to reduce the weight of the car  16 . 
     The sideplates  118  are supported along their inner sides by narrow horizontal longitudinally-extending stiffener plates  148  to add rigidity to the stub sill  50 . Additionally, doubler plates  150  and  152  are attached respectively to the sideplates  118  and end plates  116  near their intersection with each other at each outer corner of the stub sill  50 . A corner fillet doubler  154  is provided for the bottom plate  120  and a similar corner fillet doubler plate is provided for the top shear plate  52  at the outer corners of the stub sill  50 . 
     The coupler tube  58  is of conventional construction and includes a top plate  158  extending horizontally beneath a U-shaped opening  160  defined in the top shear plate  52 . The sides of the coupler tube  58  are defined by a pair of parallel longitudinal upright stub sill webs  162  extending from the end sill or plates  116  to the saddle bolster  36  or  38 , and a buffer block  164  of conventional design extends transversely between the sideplates  162  at the requisite distance from the striker plate  60 . 
     The described integrated structure of the stub sill  50 , saddle bolster  36  or  38 , and center sill  44 , together with the tank  22 , provide a railway tank car  16  whose tare weight is significantly lighter than a railway tank car of similar capacity of traditional European type construction, yet the unified structure of such a railway tank car  16  has ample strength to withstand the stresses resulting from static and dynamic loads imposed by inclusion of the car  16  as part of a train. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.
A railway tank car of light weight and high cargo load capacity in which a tank shell structure is carried on and unified with an underframe including a stub sill at each end, a pair of spaced apart saddle bolsters, each joined to a respective one of the stub sills, and a center sill interconnecting the saddle bolsters. The tank shell rests on and is welded to the center sill, the saddle bolsters, and the stub sills, so that the tank shell is an integral part of the car, and both static and dynamic load forces resulting from train operation and from cargo loads are shared by the tank and the underframe. The underframe is constructed primarily of welded flat plate, with a minimum of specially formed parts and difficult welds, in order to simplify and minimize the costs of construction.