Locomotives need lots and lots of torque at zero and low speeds. This would otherwise necessitate a huge transmission. Plus, the weight penalty for a train locomotive isn't nearly as severe as it is for any kind of flying craft.

Other commenters have mentioned a hybrid system, with a relatively small electric motor that generates just the instantaneous torque changes needed for the control system, but the majority of the power is supplied via direct mechanical linkage as in today's helicopters. That might be viable, but I'm not sure. The weight and complexity penalty for the old control scheme isn't so bad when scaled up, but is really bad when scaled down.

So a hybrid scheme (like in a Prius) might be practical, by my estimation.