RWL posted:BTW, the reason for the two long sidings is because I have two fairly long passenger trains that I would really like to have on the tracks at the same time. One is a Lionel Grand central express with the 3 3 passenger cars and the extra baggage and dining car, and the other is a set of 4 Pennsylvania Madisons plus the extra combo car, behind a Polar Express Berkshire and tender. Both are right at 80" long, and all are O-31 compatible.
I figured that, just didn't know how long your passenger trains were. SCARM says the length of the track between the switches for the top siding is just over 77" and the bottom is just over 85", so it looks like 80" might be a tight fit if you want all the switches clear. Of course you can always schedule the trains so the bottom one arrives in first and departs second or they aren't there together. 
BTW, are you buying switches with the DZ1000 or DZ2500 switch machines? I ask because the 2500 wiring requires isolation points to be cut before the frog, but I think you'll be leaving enough track to make those cuts.
Here's something else to consider if I got the space right at 114"x151". It only requires cutting the 2 upper left switches, but I'm not sure they can be cut enough for things to fit. Just throwing it out there.
