Okay, a 20 car train of, lets say 14 40'-ers and 6 50'-ers needs about 11' of track, exclusive of motive power, so your sidings in the staging yard need to be that long to avoid fouling the access turnouts, and longer if you're planning on leaving the motive power with the train in staging.
My layout is an around the room, point-to-point design with two diverging routes. One line climbs while the other drops. Both end at another staging area, one above the originating staging yard, the other below the originating yard. On those occasions when I just want to run a train for visitors, I have a short run-through track which climbs from the lower staging area to the middle staging yard. The elevation difference is 3 1/2" in 100", so a 3.5% grade. Any of my medium-size locos can pull a 24 car train up this grade, if they are doubleheaded. Unfortunately, the set-up is such that I don't need to reach into this area, so clearances are only sufficient for the trains, there is no "hand-room". Determine how much room you'd need to allow access for your hand (above the train), then add that to the height of your tallest car. An eighth of an inch is plenty of clearance for the car, and if you subsequently acquire a taller car, put a restriction on it in your "rulebook". This will add operating interest by requiring you to include routing info for that car, just like the prototype does.
A lot of my cars are older ones that don't roll as easily as the newer stuff, so my 24 cars would probably translate into more on your layout, meaning that 2 locos pulling 20 free-rolling cars should easily climb a much steeper grade than 3.5%, say 4.5% or maybe even 5%.
I just went into the layout room and discovered that 3 1/2" of height will allow me to reach over top of and grasp an Athearn 40' boxcar, as long as there's nothing on any track between me and that car.
By the looks of things, the minimum length of your gradient up from staging will be 8', and depending on how much "rerailing access" room that you require, could be longer. However, since the grade is not intended to be viewed for operating purposes, the steepness shouldn't matter. You could also have the mainline duck behind a large structure or other view block, just befor it begins the descent to the staging yard. This would help to minimize unuseable space over top of the downgrade.
Wayne
