[toddalin]

I use the reed switches to trigger 555 chips that power the relay. Then you can set the time the relay stays open with a pot.

You can also do this using a track gap using the "Tortoise Bump Accident Sentinental System" (Bump A.S.S.) that needs no reed switches and will trigger from any engine, lighted car, or car with metal wheels. I do this for my crossing where two trains would normally collide. But to everyone's amazement, they alway stop short of a collision and the second train to approach will wait for the first to clear the crossing before proceeding.

 

[toddalin]

Actually, all one needs to do is make sure that there is always shade on the sensor. This could be from a trackside structure or even overhead foilage.