The lights on the pole are what allows Train movement. In a CTC area such as Plymouth the code line provides the control to the dispatcher. In the event of a system failure all signals revert to the most restrictive aspect, generally an all stop. At this point the dispatcher has to give verbal permission to pass the signal.

When this happens things pile up pretty quick. In some cases this can mean lots of trains or it can go the other way as it did in this Ice Storm. Most trains died on law and by early morning the signal department was isolating the damage. This truck was the only CSX movement I saw in Plymouth that day!