Peak Rail has it's yard with plenty of interesting stuff.
Can you walk around the yard to view the stock or not?
Midland Railway at Butterley is fine for a full day. Dont think you can do it in a few hours because you wont manage to see the best bits. Swanwick is only accessible via the train from Butterley so forget the car. Leave car at Butterley station and just go with the flow.
Always enjoy a visit to the Midland Railway, the first time I visited I didn't realize just how much there was at Swanwick, one of the sheds is tucked away and a bit of a walk from the main complex around the station, I didn't know about it until my second visit!
The Golden Valley light railway is worth a ride too.