I am ALSO a train expert, having had a fascination with them for much of my childhood. 
It isn't a single, solid piece. I imagine that the PD must be designed a lot like the largest steam locomotives were.
[brief history of railroad engineering recap]
Everybody knows what a steam locomotive looks like, right? Big round boiler body, with big wheels and a drive shaft underneath? Possibly a few smaller wheels in front or in back?
The first locomotives only had four "drive" wheels (the big ones that are driven by the actual engine). Later, more powerful ones began adding wheels two at a time, so they gradually went from four to six to eight.
Well, when they tried to run engines that had eight or ten drive wheels on narrow, mountain tracks that twisted and turned, they ran into some problems. The engines were getting STUCK in the curves, because they were so long and the drive wheels had to be one rigid unit for the system to work.
They got around this with an innovative design. I know there's a word for it, but it escapes me at the moment. Basically, they took the body of the engine and put a hinge in the middle. Half of the drive wheels were mounted in front of the hinge, half behind it. Each set of drive wheels had its own, independent drive shaft.
Using this system, they were able to build powerful locomotives with 8, 10, and in later ones 12 or 16 drive engines, but were still able to fit around tight curves.
[/brief history of railroad engineering recap]
*deep breath*
That HAS to be exactly what they did when building the Prairie Dog. The thing BENDS when it goes around a curve. The front set of wheels, the middle area where Big O is stored, and the rear set of wheels have hinges between them. Heck, you can actually see it in action like this in some episodes (Beginning of Enemy is Another Big, I think).
Of course, even with this it still probably wouldn't fit around the curves in a modern-day subway, which is why I maintain that the PD runs on tracks that were built exclusively for its use, and are underneath the old subway lines.
It isn't a single, solid piece. I imagine that the PD must be designed a lot like the largest steam locomotives were.
[brief history of railroad engineering recap]
Everybody knows what a steam locomotive looks like, right? Big round boiler body, with big wheels and a drive shaft underneath? Possibly a few smaller wheels in front or in back?
The first locomotives only had four "drive" wheels (the big ones that are driven by the actual engine). Later, more powerful ones began adding wheels two at a time, so they gradually went from four to six to eight.
Well, when they tried to run engines that had eight or ten drive wheels on narrow, mountain tracks that twisted and turned, they ran into some problems. The engines were getting STUCK in the curves, because they were so long and the drive wheels had to be one rigid unit for the system to work.
They got around this with an innovative design. I know there's a word for it, but it escapes me at the moment. Basically, they took the body of the engine and put a hinge in the middle. Half of the drive wheels were mounted in front of the hinge, half behind it. Each set of drive wheels had its own, independent drive shaft.
Using this system, they were able to build powerful locomotives with 8, 10, and in later ones 12 or 16 drive engines, but were still able to fit around tight curves.
[/brief history of railroad engineering recap]
*deep breath*
That HAS to be exactly what they did when building the Prairie Dog. The thing BENDS when it goes around a curve. The front set of wheels, the middle area where Big O is stored, and the rear set of wheels have hinges between them. Heck, you can actually see it in action like this in some episodes (Beginning of Enemy is Another Big, I think).
Of course, even with this it still probably wouldn't fit around the curves in a modern-day subway, which is why I maintain that the PD runs on tracks that were built exclusively for its use, and are underneath the old subway lines.
Steam engines have side rods and valve gearing.
