Can I build a faster LEGO train motor? I was going over LEGO Power Functions train motors, and I was not satisfied with the speed. Speed seven is fast enough to derail the train on the curve, but not fast enough realistically. I am thinking that if I open the motor and put in bigger cog wheels, it will hopefully make my train go fast enough (I'm trying to get at least 10 mph.) Will this work? Where can I get bigger cog wheels?
Mathematically, the largest driver wheel you can buy for LEGO is Big Ben's XL driver, with a diameter of 36.8mm.  To reach 10 mph, that wheel will have to spin:

36.8 mm diameter -> 115.6 mm circumference -> 7.2 x 10^-5 mi/revolution -> 2320 RPM (!!)

This means that you have to gear up a motor to run at 2320 RPM.  An XL PF motor with no load spins at about 220 RPM.  I just don't think that's going to happen.

And if your goal is to use the smaller wheels, there's no way to accomplish what you want with LEGO.

Don't forget, the TGV (and other high speed trains) are incredibly complex and well engineered machines.  It requires a specific track layout, curve radius, rail width tolerance, surface roughness tolerance, and we haven't even reached the train itself.  

Honestly, accomplishing a scale-equivalent speed using only LEGO bricks would lead to a revolution in transportation design, because if you can do it with LEGO at L-gauge without blowing up, you could easily do it with steel, plastics, and composites at people-scale.