David:
As Chiloquin Russ mentions, a lot depends on your railroad, i.e. mainline, branchline, etc.
For steam locos, they needed facilities for fuel (coal, oil), water, sand, lubrication, and an ash dump (coal burners) as a minimum. Many lines had coal and sand combined in a larger tower. Water tanks could be trackside (spout attached) or in the distance with a stand pipe for watering the locos. Trackside sheds would house lubes, kerosene (for lanterns), fusees, flags, waste packing, and perhaps something needed that was totally local.
Engine houses ranged from one-stall houses with no equipment to multi-stall affairs that could do everything but the most difficult repairs.
Pick your operating scheme and choose your structures accordingly. It's been frequently mentioned that you can find a prototype somewhere for just about anything. You're the CEO/COO...have fun!
Almost forgot the call boxes (for sure) and a scale house at the modest-sized yards.
Neil