This section discusses how the railroad preemption circuits discussed in Chapter 4 should interface with TxDOT approved traffic signal cabinets. Traffic signals with railroad interconnection should use the standard TxDOT traffic signal cabinet, which is a Type P (44”W x 26”D x 55”H) that conforms to NEMA TS 2-2016 Traffic Controller Assemblies with NTCIP Requirements - Version 03.07 or latest Standards. Other NEMA cabinets may be used, but they shall accommodate the preemption related equipment and operations discussed in this chapter. The standards presented in this manual also apply to legacy style traffic signal cabinets that conform to the NEMA TS1 specification. Likewise, this preemption standard is compatible with the NEMA ATC traffic signal cabinet specification. However, the use of an ATC cabinet at locations with railroad preemption requires a design exception.

The primary input method of railroad preemption information from the railroad to the traffic signal controller is through the railroad interface which passes the information to the preemption input terminals located on the traffic signal cabinet main panel. When timing the railroad preemption warning time for the grade crossing, the standard TxDOT railroad interface sequences through multiple preemptors in the traffic signal controller while executing the preemption plan. The designer and traffic signal technician implementing the railroad preemption shall account for the use of a minimum of six preemption input terminals for railroad preemption. Six preemption input terminals may be necessary depending on the requested railroad circuitry.