Nigel,
That is one big sukker of a turntable! i have pushed a narrow gauge table by hand; i do not think you can do that with the new one there!
Ted Miles, IRM Member
You will want a 400-foot-diameter space to allow for a full roundhouse to be built eventually. (135 feet on each side of the turntable, to handle the *largest* possible thing you could turn on the turntable)
I think it makes sense to build the roundhouse basically one or two bays at a time -- treat each set of bays as a separate construction project, and count the track and the building separately, much like the numbered yards and carbarns.
But leave enough space for a full all-the-way-around roundhouse. In 40 years you will be glad you did.
The detailed design is being prepared. One of the big variables is an assessment of the ground underlying the site. This will determine the extent to which piling will be required.
The plan is much along the lines that you suggest. The expectation is absolutely that bays would be constructed in phases. It is unlikely that a phase would be as small as 1 or 2 bays. Bear in mind that each phase must initially stand on its own so has a temporary end wall, to be removed when more are added, that is additional cost that has no value once the next phase is added. So it is likely more cost effective that the phases are somewhat bigger.
Also, as regards the overall size. It is necessary that all stalls are of a length to accommodate the longest locomotives. So, if it fits the site better, the roundhouse need not be a perfect circle.
Nigel
those additional "temporary" walls you speak of, could, and should, have a long term use as firewalls to slow or prevent the spread of a fire from one section of the roundhouse to the adjacent sections