Relay question.

Two most common options are infrared detection or insulated outside rail triggering.

Here’s a link to a highly recommended maker of infrared detection/activation devices Azatrax

They have very good installation instructions.

Ditto to Steve H.  I use Azatrax MRAPR 12V DPDT relays to control four 153 block signals on my 027 layout.   Their website has good wiring diagrams.  You're going to need a few pieces of track with insulated outside rails, and a pile of insulating track pins.   

It took a little fine tuning (and a couple of train wrecks) to get the "coasting" distance figured out so the trains don't roll through the red light.   It was a lot of fun getting everything to work. 

This may not be exactly what you are looking for but it might be.  This is for the train detection light (red=train in zone; green= no train in zone).  The rail between the two X's is an insulated rail.  Sorry about the mess with the lines in the bottom diagram: as noted, in the top diagram I have two wires that need reversed - that's what the bottom diagram is trying to show.

Here's a diagram that was posted on the OGR forum quite a while back by ????.    There's a big difference is how things are powered.  I prefer to power the relay with a small DC trannie as I showed.