Making axles rotate will not affect its performance. The fact that they are a part of the train will not increase the amount of frame rates you get whereas a train with no axles at all will run a lot better.
If you still want to stop them from rotating, in the Animated file there is a line of code that goes something like this:
[object]
States = Trucks\Wheel.csv
Position = 0, 0.432, -7.04
RotateXFunction = value + delta * speedometer / 1.55
Open the file in notepad and delete what's in bold and your wheels will stop rotating.
BStyles