Locomotives actually need to be a certain weight: you see this with "slug" locomotives, which have no diesel engine and need concrete or steel plates to make up

Locomotives actually need to be a certain weight: you see this with "slug" locomotives, which have no diesel engine and need concrete or steel plates to make up the difference in weight. This is because the maximum torque that can be applied before you reach wheel spin is partly a function of the weight applied to the driving wheels. So in order to pull the weight that a diesel electric locomotive can, you'll probably need to add weight to the electric locomotive, and you may as well add it with batteries.