I would so some tests with typical train formations to see what the maximum gradient is viable, not forgetting that most of the rise is likely to be on a curve. You need a minimum of 3 inches clearance but that's assuming there's no upper baseboard framing to clear: 4 inches would be a better minimum, especially if the upper board has points with motors below the baseboard, and there'll be wires too. And the upper section needs to be removable not just to access the lower tracks, but also to deal with any upper board problems such as wiring and point motors.
Now rising 4 inches at 1-in-50, for example, needs a 200-inch slope - or nearly 17 feet. And you need this both down and up. On my layout I have a scenic section on one side above a storage yard, and although I have more space (28ft x 8 ft) it was still a struggle. I needed no steeper than 1-in-70 for my 8-coach steam-hauled trains. In the end I had to split the difference: the scenic section rises and the yard sections falls in equal measure - about 2 inches each. Diesels tend to be better than steam in load-hauling terms. My Bachmann 08 diesel shunter easily out-performed my King Arthur, for example. I have 4 inches clearance which allows for 1-inch cross-bracing and just room for PL10 point motors with PL13 accessory switches. My first test came to grief when the train cleared motors and framing but was lassooed by a dropper wire that was hanging too low! Now I've fixed them all up.
Also removing weights from coaches can help reduce the load. My King Arthur would only take 6 Bachmann Mk1s up the gradient, but once the weights were removed from the coaches it takes the regulation 8. Another point to bear in mind is whether you need to start the load on the gradient or whether trains will already be moving well by the time they hit the slope. Signal checks at the top of the slope are to be avoided.
Next problem is working out how to track what's in the yards, especially with DCC. You not only need to set the points correctly but also need to know which loco is any given road. As I have computer control I'm thinking of using a webcam - you can get them with LED lights for around £10. Alternatively I may assign certain trains to specific roads in the yard. I only finished the upper section yesterday so it's still work-in-progress.