I know nothing of trains, but, someone once told me, that some of the long freight trains can have 2 diesel engines pulling, and one in back pushing. Apparently the weight or volume of such trains are helped by having an engine push as well as pull.
One of the big reasons for multiple engines-one in back, is because a “chain” wants to go straight. Have to keep the train tighter or it wants to derail. Not so much on the great plains since the rail system is pretty straight and not curvy or hilly.
Then, there is the issue of the train breaking apart going up a hill and just over the crest of a hill and speed braking going down a hill.
They have to really watch it when half the train over over the hill and going down the other side, and the rest is back on the uphill slope.
The pushing engine helps relieve the strain on the couplings on a long train. They would just put it in front with the other engines, if all they needed was the power.
I’ve seen really long trains with multiple engines front back and middle. Rail fans post them on YouTube.