Um, sorry Sammy, that's what they call perpetual motion and it won't work. See all the other posts here on the same subject for all the reasons it wont

Hyenuf is certainly right, but ironically, when I worked on trains, we were at a loss to provide electrical power to run sensors and electronically controlled braking systems on the cars. A car can be up to a mile from the locomotive and only certain cars needed temperature, load, g-sensors, and a mile is alot of wire, so yes, we sometimes used 'axle generators.' The power does come from the locomotive, but there didn't need to be a wire. I'm sure it was much less efficient than a wire though.

Generators on car wheels would take way power from the movement, power that would need to be overcome by the locomotive generating more power, spending more fuel doing so.

Now, it might be practical to use wheel generators to power smaller things on freight cars, if locomotive electric power isn't distributed the whole train.