Roundhouse
Used for locomotive storage and minor daily repair andmaintenance.
Maintenance Shed
Performed total disassembly (which took about a month) and major engine overhauls. Equipped with in-house blacksmith and machine shops capable of repairing and fabrication.
Sanding Facilities
If your layout will have some rough terrain, sanding facilities will be needed. Consider a sand pit and/or sanding tower. Often sand arrived at the yard wet, so a sand drying facility was used.
Ash Pit
"Clinkers", the byproduct of fired coal, and ash were dumped here.
Water Tower/Watering Facilities
In later years...water, coal, and sanding towers were combined into 1 structure.
Coaling Tower
Depending on the era you're modeling, these structures varied greatly in appearance and materials.
Fueling Station
Not all steam engines used fuel. Some even used propane.
Turntable
Self explanitory.
Click me - Nice pics of a model RR service area