I had several stations on a looped track, and controlled the train position by switching on the power to the motor for a certain time.
At first I used a photodiode taped to the CRT monitor (and controlling the motor by lighting up a square in the corner of the screen), later I upgraded to controlling a relay via the parallel port.
Wiring the serial or parallell port to something wasn't hard. Instructions for bit banging them where generally found in the library (paper books). Serial had weird voltages but was near indestructible. Parallel was TTL just like anything else digital, so some 74 ICs got you mostly there.
The era where you basically needed to solder if you wanted a computer was just over, so electronic knowledge was still easy to find.
We're all engineers now, the box is treasured and still works. Finding a PC with a parallel port has become challenging though.
Plus there are a bunch of off the shelf solutions on Amazon (YMMV).