A recent thread on "balloon tracks" led me to create this post about the construction and function of hidden staging yards on my railroad. Following concepts in Frank Ellison articles - the railroad is the stage, the trains the actors, and the timetable/train orders the script. When not performing a function according to the script I wanted most through freight and passenger trains "off stage" between runs - thus the requirement for large capacity hidden staging yards. My railroad can be thought of as a multi track dog bone design. It features a 375' multi track main line run with hidden staging loops at each end., and the Altoona yard and engine terminal in the middle. Following PRR practice through steam powered freights pause at Altoona for crew and locomotive changes. The Pittsburgh Division's east slope helper operation add spice to the mix. The railroad also features 2 branch line connections to the Middle Division main - their function and construction will be off topic for this piece.
To find space for balloon staging tracks I used a layered design influenced by John Armstrong, and lessons learned on my prior railroad (see March 2000 MR). The east (Middle Division) staging is under the Horseshoe Curve scene at a base elevation of 47". The west (Pittsburgh Div.) staging is suspended above other elements of the railroad at an elevation on 76". Crossovers on the 3 and 4 track approaches to the staging yards are arranged to allow multiple trains to enter and leave at the same time.
The first 5 photos show construction of the suspended Pittsburgh Division staging yard. For ease of installation it was built before the Altoona yard area located below it. The staging structure was built on the floor using 3/4" plywood in 7 sections. It was then hung one section at a time from the floor joists using 1/4" threaded steel rods passing through 2x4 cross members. Homasote roadbed and flex track was not installed until the entire structure was cross leveled in place using 4 nuts on each rod. Once the track was down I attached 3/8" bending plywood (a.k.a. wiggle wood) sides to the plywood base to hide and protect the trains. The Horseshoe Curve scene and the tracks climbing up to Pittsburgh Divisions overhead staging tracks could not be built until the lower level Middle Division staging loops were in. With layered railroad the construction sequence matters. 
Now on to building the Middle Division staging - far easier than that for the suspended loops. The photo below shows the 5 track Middle Division staging yard in the foreground with the yet to be connected overhead staging in the distance. On the left is the Altoona yard.
The next shot shows construction of the Horseshoe Curve directly above Middle Division staging . The curved stud wall is being erected to support a double sided backdrop between the Horseshoe Curve scene and the single track coal branch on the opposite side.
Here is the finished Horseshoe Scene. In the foreground you can see the outline of 2" foam lift out section that provides emergency access to the Middle Division staging tracks below.
And the final shot shows MG interlocking and the tunnel face connection to Pittsburgh Division staging.
I should add that the hidden trackage is monitored by track occupancy detectors and home security monitors. After 11 years of construction the track is done and the scenery is at around 80%. In 2018 I hope to put the railroad to the test by hosting several full crew operating sessions.
