Electric Tow Vehicle (Tesla)

We know that locomotives use diesels to power a generator to power the electric motor. We also know that each diesel engine has a sweet spot for power and fuel economy. Why could we not have a truck that "idles at 1800 rpm" running to power the generator for the electric drive motor. Forget batteries. We would have the electrical motors max torque from rpm 1. I personally want the economy of a diesel with the instant torque of an electric motor. Now if it looked like that....maybe not.