In some area's in Surface Tension, such tanks did show up. In fact, there was a func_tank present at the place where the vehicle would stop, and it was only activated when the vehicle stopped there. With env_renders, it was then made visible (and the turret that moved, wich was actually a func_train, was either killtargeted or rendered away. Probably killtargeted as it shouldn't catch bullets so to say).
So, that's indeed faking, as hazardous! already said.
But to use such things in a CS map... I guess it's far too advanced for now. Maybe it's possible with a lot of entity trickery, but I think it's better to wait for CS2 and await it's possibilities. Nice idea though.