Ray sent me a MTH RealTrax ITAD 40-1028 to mess with. While I doubt anyone would attempt to repair or modify such an inexpensive item, I nevertheless dis-assembled it for learning - and here's my take on the schematic made by tracing out the double-sided circuit board. There are missing/unlabeled surface mount capacitor values which I did not measure but not an issue for anyone skilled in the art.
Internal photos
I do not know if the ScaleTrax ITAD (45-1028) is of the same design, but what I found interesting the low-angle or height of the beam that is meant to bounce off the passing train. Above photo shows the sensor window and the emitter on the right side and detector on the left side. You can see the purple-ish glow of the IR emitter (LED) which hits the passing train at just about the top of the wheels for a diesel engine or passenger/freight car. Yes, the beam does spread out a bit but the reflection surface is not half-way up a boxcar side. This goes to the comments about painting or modifying the surface for better reflection/detection of dark objects.
The other "mystery" which I've never found documentation is how it connects power to the relay common. Answer: the right terminal of the AUX POWER INPUT is internally wired to the output relay COMMON terminal. So this is what is directed toward the output NC terminal (when ITAD not triggered) or to the output NO terminal (when ITAD is triggered). What this means is (when using AUX POWER) you can choose whether to switch AUX HOT or AUX COMMON thru the relay. There are some applications where this may come in handy. You can also use DC to power the ITAD though it needs about 13V DC.
When powered from track power, it is the outer rail that is steered to the NC or NO terminals.
As for operation, this unit is indeed very sensitive to ambient light. The unit I was evaluating would trigger (without a passing object) with what I consider a moderately lit room irrespective of the RANGE setting. So consideration must be given to where to locate it relative to windows, overhead lighting, etc.
