Hi GS4,
I think it kinda depends on what sort of trains your yard will be servicing.
For a yard for commute (suburban) trainz you would at least want a place to store the fleet during non-service hours, cleanout tracks to clean the interiors and turn the seats for the run back up the line, and a RIP (Repair in Place) track for minor repairs to equipment, which could be shared with general service facilities if the terminal includes a nearby servicing facility. At one end or the other you also would want a wash rack.
If your yard will serve intercity trains I think you would need at least the following for a serious coach yard.
1. Everything mentioned above, plus
2. A building and spur to service diners and bar cars, including cold storage for perishables, facilities for food pre-prep work, and a bakery, and loading ice and cooking fuel.
3. Some facility for turning observation cars, such as a wye, or entire trains or at least first class compartments and sleepers, a loop if possible, if your route features scenery you want to show off that is only visible from one direction.
4. Mail building and spur for loading RPOs and sealed mail cars.
5. Building and spur for Railway Express Agency for loading express packages.
6. Some sort of parking spur for private cars, including access to steam and electricity.
If you search Google you will probably find a number of maps or diagrams of important terminals, such as Washington DC or one of the Chicago or New York stations or terminals that will give you even more ideas. You are definitely on the right track, though. Passenger train operations are a lot more complicated than dashing to the next town at 80 mph, and are often overlooked.
Have fun with it.
Bernie