A moderately complex train detector using an infrared LED and an infrared detector.
Since this Web page was originally posted, I heard from several people - and found about it personally - that Radio Shack had changed the parts in the infrared emitter/detector kit, part number 276-142. Unfortunately, each part was not clearly labeled; I even found one kit (example 2 below) where it's not easy to tell the emitter from detector. Basically, check the parts out closely, and match them to what I've designated as either style 1, 2 or 3.
If anyone runs across a new style, let me know. If you want, we can decide to send the new part to me, for debugging.
Click on the pictures above, or for Style 1, choose
this link, Style 2, choose this link,
Style 3, choose this link.