I gather you are wondering how the locos all pulled together or whether there was some loss as one loco "fought" another; I'll let others answer about how those locos are each controlled. There was never any real steam MU control (probably some one tried to do it) so then each crew would try to co-ordinate so the locos didn't fight each other--much. But it would be inevitable there would be losses of energy as output differed even temporarily from each unit for a host of reasons; we know this happened. But trial-and-error showed early on that double-heading (and more) worked despite the waste of some power. Same with compounding and its back-pressure "problem"; the Schmidt superheater got more bang for a lot less $$$ and compounding disappeared from the railroads. Electric and later Diesel MU required fewer crews and as crews became more expensive that cost told.
Just like in the rope scenario above, think of the four people pulling on the rope. They all have different strides, but they are all pulling at the same speed. Some are just taking more steps than the others.
Dave
Multiple locomotives in a train simply add together their pulling power, whatever that might be. All locomotives may be exerting the same amount of force from each unit, or different amounts at any given time. That is why diesels (and steam) can utilize differing models and-or generations of locos in any given assemblage. In the diesel age, we refer to each loco as a "unit", and a group of units at the head end of the train is called a "lashup". Rear end helpers are called pushers, and mid-train helpers are call "DPUs", for Distributed Power Units. They can be radio-controlled from the head end, or be controlled (rarely) by additional crew members on board those units. The effect of adding the various units' pulling power together eventually exceeds coupler strength, and that is the purpose of not putting all units on the head end of very long & heavy trains, especially where steep grades must be climbed. In North America, any grade over about 2% is considered steep.
It has been my understanding that when power is distributed anywhere other than at the head end and all of the power is controlled by the engineer at the head end, all of the units, no matter where they are in the train, are described as "DPU"--and a helper is an engine with an engineer who operates that particular engine over a particular stretch of track.
Johnny
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