A track warrant is a set of instructions issued to a train crew authorizing specific train movements. The system is widely used in North America. The warrant is issued by the train dispatcher and delivered to the train crew via radio. The train crew copies the instructions onto a pre-printed paper form and reads back the warrant to ensure that nothing was misunderstood. Operation Track warrants are issued granting main track use between two named points (i.e. milepost sign, station, and/or any fixed physical point, such as a switch). The dispatcher may also issue time constraints (known as "Box 6" on a standard form), although the track warrants remain in effect until cleared by a member of the receiving crew. Track warrants are sometimes used in conjunction with a block signal system to provide rear-end protection against following trains. Track warrants usually allow a train to move in one direction only—a "Proceed" instruction. Sometimes a train may also be given authorizing movements in either direction, called a "Work between" instruction. However, only one train can have a section of track at one particular time if moving in the same direction. If there are two trains moving in the same direction, the leading train must give up the track before the trailing train can obtain a track warrant for that territory. For example, if Train 56 has a track warrant to proceed in the northbound direction from MP (milepost) 14 to MP 77 and there is a train following, then Train 56 must give up a portion or all of its warrant before the following train can be issued the track that Train 56 has already cleared. Many times this is accomplished by the dispatcher asking the leading train for their milepost location and then issuing a warrant up to that point to the trailing train. Continuing the previous example, if Train 56 is clear of MP 50, the dispatcher can issue a warrant to the trailing train up to MP 50, but not beyond it. Once a train has received a new track warrant or has left track warrant territory they will release the entirety of their warrant back to the dispatcher. Maintenance of Way crews receive track bulletins (usually called "Form A", "Form B" or "Form C") in order to perform track work that would otherwise be interrupted by passing trains. This is done by providing the maintenance crew