Kris,
A model number would help, and you can also Google it to find more info.
As far as the 555 timer goes, I don't see the connection with the traditional semaphore unless it was used to limit on-time for a smaller display layout that might leave the train setting on the activation device for extended periods.
However, on a larger loop it could be used to hold the RED indication for a longer period after a short train passed.
These are usually used with an under-track contactor or an insulated-rail.
This document shows typical hook-up instructions Lionel #159
But look up your particular model.
Dave
