.......lets see.... You could put two VERY thin pieces of wire up through the track so they are almost touching eachother and just brushing the axels... when they touch, you would make a light flicker. Course you will really need to tune it so it does not knock the cars off the track.
I haven't checked on the cost to build them lately, but the Twin-T detection system designed by Linn Westcott in the late 50's is relatively simple, and there have been a few offshoots and improvements over the years. Peter Thorne describes them in one of his books, Easy-to-Build Electronics Projects for Model Railroaders
I found the link. He makes them per order. He's a Ham Radio Operator too that I have talked with him on the air. They are very simple to use. You can use them with Sunrise and NJ Internation signals. Others too. If you use lamps instead of LED's, you need to install a relay. Very, very, very simple to use. They are small enought to use on Nscale too.