A very nice set of answers to some difficult planning issues. But I have a question (mainly because I'm struggling with it myself). As it stands, this design is no problem for diesel power since they traditionally run with either direction as forward. However, steam locos tend to to have one direction forward, only going in reverse where they must, often along branchlines. In a fairly urban setting with many connections to other systems, I wouldn't expect to see mainline steam power pulling in reverse. So how could these big steam units be pulling a load in one direction and later come back in the other direction, w/o a turntable or a wye? Is there no choice but lifting off?
Back in the day I suspect the eastbound engine would require fueling or servicing as they did every 100 miles or so. After servicing they would either continue eastbound or use a turntable to turn. For the layout, don't forget the possibility of having a hidden turn loop. This can be a very simple bypass piece of track that connects 1 part of a loop to another. Another option is to have a turntable in the staging. Doesn't have to be fancy. But if you are running a dubbel header in consist it will be a little more cumbersome to turn them. Personally I would have 2 sets of engines. 1 for east bound and the other for west bound. The compromise would be that the same engines always run in the same direction. So be it! Maybe do some extra research if they really didn't run backwards. It can help to narrow down the exact stretch of track.
