What is the main train?
The beauty of the diesel electric locomotives is that no matter how many engines are in the consist, only one engineer is required to control all of them. This is why on the US and Canadian railroads you typically see multiple engines on the train.
Often, on very long, heavy trains you may see engines in the middle or the end of the train. These are called Distributed Power Units and they are unmanned. The engineer in the lead locomotive controls all the engines in the train by remote control, either by radio or by hardwired trainline.
There are many advantages for using Distributed Power Units. First is the reduction of the draw force on each coupler which permits operation of longer trains. On the undulating sections of track where one part of the train may be climbing a hill while another part is descending, an experienced engineer can manipulate relative power output and dynamic and air braking of different locomotives to minimize coupler run-in and run-out.
On the long trains when an engineer applies air brakes from the lead locomotive it may take several seconds for the change of air pressure to reach the last car in the train. Distributed power allows a more uniform application of brakes throughout the train.
On the long steep climbs heavy trains may need to use helper engines. These typically couple to the end of the train. The helpers usually do have a crew who, once the train reaches the top, uncouples from the train and run the engines back down the hill to help the next train or couple onto the downhill train to help with dynamic braking.