Here is a posting from somewhere that I saved a while back, maybe it will help:
This is one area where almost everyone has to make compromises. A full servicing facility just takes up too much space. The "typical" inbound prototype sequence of events (for steam) was:
1) Dump ashes
2) Wash down
3) Inspect
4) Refuel (oil, coal, sand, water)
5) Turn and send back out or store.
In the real world, engines were refueled immediately after inspection (unless they needed extensive maintenance) before they were put away, so they would be ready to go on a moments notice. Assuming, that is, that the prototype had the real estate -- many suffered the same lack of space that we do.
If you have both an inbound and outbound lead, then refueling could be moved to the outbound track. If you have only one lead, it really does not make much difference, since you can skip steps depending upon inbound or outbound.
On my layout I wanted to get the inbound activities as close as possible to prototype, so I have ash pit, wash platform, inspection pit (all from Diamond Scale) all on the inbound lead. However, this takes up a whopping 36" before the turntable in HO, so it really eats the space.