Ron, you're really rolling now. That Union Station project sounds like fun. I like the idea of the trolley line and the autoreverser you linked to is a good one, much more realistic than the ones that just cut power and reverse power.
As far as the roundhouse set-up, I'm far from a steam expert but I remember the way the NKP did it back in the mid 50's. There were two types of engines, "cold" and hot". The cold engines were ones coming in off a run and were either scheduled for maintenance or had no schedule runs in the next two days. They dumped the ashes on the way into the roundhouse and got the firebox cleaned out with hot water. Another crew used steam to clean the firebox also if the engine was due for maintenance.
Hot engines were ones that came off a train and were due back out in again any time within a few hours to a day. Engines running local service wouldn't drop the ashes until the ashpit was nearly full or full enough that it wouldn't have enough capacity to complete the next run. The big engines like the Berkshires always dumped the ashpit whenever they were at the roundhouse. The hot engines had hostlers and watchmen that kept the engines stoked up to keep enough steam for the next run. The Berkshires would even dump the ashpit on the way out if they had been kept hot for more than the usual overnight stop.
Coaling was done on a case by case basis. Again, the smaller engines in local service would always coal up before going out of service for the day if they were hot. The Berkshires had a huge coal capacity and would only coal up if the road foreman calculated the engine wouldn't have enough coal plus reserves for the next run. The NKP didn't like hauling around their own coal for free if they could avoid it.
The engine track always has the ash drop pit and washout pit first since it was both dirty and dangerous to have all the hot ashes near the coaling tower. A cold engine always had enough steam in the boiler to get to the coaling tower and roundhouse even with no coal left. A hot engine would drop the ashes and then get a washout before coaling up to make sure there were no hot clinkers left in the engine. Most engines would coal up on the way back to the main to lessen this issue but some had to coal up because they were nearly empty before they got to the roundhouse and didn't have enough coal to keep steam up while they were off duty.
It's also important to remember that two types of coaling facilities were needed. One was in major yards on the engine service tracks. In major yards, there was also another coaling tower for through trains so they didn't foul the yard leads or engine tracks. Imtermediate or trackside coaling stations were also needed for any Class 1 roads. These had the famliar water tank and coaling tower and were usually located on a siding at strategic points om the main. The Nickle Plate Historical Society at
http://nkphts.org/ has some really good pictures of steam servicing facilities. Since they and the N&W ran steam into the late 50's and even early 60's, their facilities were well maintained right up until the end.
EDIT:One more thing I forgot is that NKP did not have roundhouse water towers in the late years of steam. They had standpipes at each track in the roundhouse and the boiler tanks were filled there. This was another NKP innovation to keep engines from lining up waiting to get to the roundhouse while other engines were taking on water.