In your space you'll be building a smaller terminal; if you're doing transition era, just include a roundhouse and steam servicing facilities (coal tower, ash pits, water, sand) and add diesel fuel tanks and fueling stands on one of the approach tracks to the turntable. If you have any other tracks running alongside the approach and servicing tracks, some diesels can be parked outside, or run into the roundhouse for light servicing.
The prototype might eventually upgrade and replace things like sand towers, water stands (passenger units with steam generators still need water, but filled using a hose setup, not from a huge spout off a small water tank) and tear down obsolete coaling towers and fill in ash pits, but wouldn't be likely to completely rebuild the entire complex, just add the bits they need. Unless the structure needed replacing, they'd keep using the existing roundhouse and service buildings.
So for your facilities, you'll want to include
- coal tower
- ash pit
- water tower and stand pipes
- sand house & tower
- turntable or wye for turning steam engines
- enginehouse
- diesel fuel tanks and fueling stands
- misc. little tool sheds and shacks
Coal, water, sand and fuel would be located on the outbound track(s) of the turntable. The ash pit would be on an inbound track.
Optionally include additional tracks parallel to the TT leads to use as diesel parking/ready tracks. Depending on how much traffic you're intending to have coming out of your terminal tracks can be combined or added to handle more engines or less space. Also consider a "RIP" (Repair-in-place) track for minor repairs to freight cars.
MR had a really great article on modelling a steam servicing facility about ten years ago; I've got a copy of the issue, I'd have to look through my stack to see which one it was. As diesels came on the scene they'd just add the required bits to the existing facilities.