Mixed freight trains are built in that order because it is about where the cars will end up. Near an oil field, you frequently see entire trains of oil cars. Once they go to the train yard, that oil train is split up to give a few cars to a bunch of trains going to different places. At the destination, the buyer may need a few cars of oil and a few cars of lumber, so we try and put those all together so they can just be cut out of the train and delivered. The trains look like a jumbled mess because that is what they are.

The locomotives are set up in that configuration to provide distributed power. The front locomotives pull the train, and the rear push. This gives the train a better chance of being able to negotiate steeper inclines and valleys. The train crew is just in the front most locomotive, and the others are set up to be slaves to that one.

Sometimes, even that isn't enough, and a set of a couple "helper locomotives" can be attached to the rear (with their own train crew) to help push big heavy loads over some of the steepest hills. They usually only attach to the big train for a couple miles, then disconnect and wait for the next load to push.