How do I find a schedule of trains coming by on a certain track?

Only a psychic can say for sure..

If it's passenger trains then the railway company concerned will have a published timetable.

If it's all trains, i.e. passenger, freight, maintenance, light engine workings, inspection tours, etc. then these are shown in the Working Timetable which is the one issued to the railway staff. It is not normally available to the public but if you ask the railway operating company nicely they might let you have a copy.

Failing that you could always sound out the local rail enthusiasts' group as they are bound to have people watching the line who will know the schedule of trains that pass.

Depends on where you are. Being in the UK I love this site:- http://www.nationalrail.co.uk/ldb/

You type in the name of any station and you can see exactly what is happening in real time. Just like watching paint dry.

Well, there's always the Braille method where you park on the track.......

Yer only gonna do this once-Pay close attention.