Modern passenger trains have motors spread throughout the entire train. It allows them to accelerate better than is possible with a dedicated power car. This is because multiple wheels have more contact with the track. Modern diesel trains have multiple smaller generators spread throughout the train rather than dedicated power cars. Both these approaches give better modularity as the power availabile scales with the length of a train.

The efficiency gain in splitting trains is mostly related to reducing the number of drivers needed. It also reduces the demand on track and platforms that may be congested. Both these things could be solved through better automation. Have the train driven by a computer and automate signalling to allow trains to run closer together.