Note that we can now easily fit 2 trains, and possibly more, on a line that previously could hold only one. This stands for the number plate on the signal and is an indicator that the signal is a permissive signal , and that it is not an absolute signal . The purpose of the automatic signals 181 through 217 is merely to space the trains apart, not to hold them at a particular location. With no reason to do so, there is no reason why any train should have to stop and wait for a signal to clear. Thus was born the permissive signal.
Railroads vary in terms of the degree of vigor they apply to such signals.
When implemented in this way, a full stop was required, sometimes with a brief waiting period, before the train was permitted to proceed at a slow, restricted speed. Other potential causes of a red signal include switches not properly lined for the main track and broken rail. Yes, the train was crawling, but at least it was moving. Chances were good that the red signal was caused by another train, and when out of the way, a better aspect would be received at the next signal. Identifying a Permissive Signal Most railroads identified a permissive signal by attaching a number plate to the mast or otherwise near the signal.