Henry_Best:
I was thinking of making my own detector by mounting the IR LED and IR detector in 5mm ID brass tubes with the tops of the tubes level with the track bed and the LED and detector some way down the tubes (think optical rangefinder). That way, I can make the detector more sensitive and focused on a narrow area.
Only the detector needs a tube if that. Put it behind an aperture. In the mid-80's at a Trenton Fest I saw some guys who made a scanner out on a dot-matrix printer. They mounted the light sensor in a pointed pen barrel and got recognizable images. The IR is just for illumination.
Depending on train speed you may be talking 1" long strips. You might try printing flat black stripes on silver reflective material and see if they have that on sticky-back sheets.
I've had Office Max laser-print sheets with strips of gradient and blocks with apertures on overhead projection sheets for maybe 50 cents. IR on one side, aperture and detector on the other I got analog of the strip position for a home made linear pot.