I have seen many photos of helper locos, especially from the steam era. Often they ran doubleheaded, sometimes as pushers, sometimes midtrain, and sometimes combinations. What were the pros and cons of using helpers in the various locations. How did the locos coordinate their speed so the pusher wasn't pushing the cars faster than they were being pulled by the lead engine. It would seem to me that this might cause derailment. Sort of like a large scale version of bump drafting.
Exactly how steam-era helper operations were organized was influenced by a number of factors, including (but not limited to) general company policy, strength of coupler knuckles and drawgear, track configuration at the top and bottom of the helper grade, even the whims of legislative bodies.
Doubleheading was preferred for passenger helpers, since a pusher just might push a carload of people off the track! Freights were more likely to get pushers - but they might have to go in front of the caboose. SP, especially, seemed to like to distribute their power (possibly because they had the most horrendous grades.)
When pushers were used, it was because the head-end power couldn't take the whole train up the grade by itself. Neither could the pusher! Somewhere in the train there would be one car that was being pulled from the front and pushed from the rear. It wouldn't always be the same car, but as long as it wasn't the pusher locomotive or the road engine's tender there wouldn't be any problems.
Pre-radio, the road engineer and the helper engineer would communicate by whistle, which is why there would be a lot of whistle talk just before the train got rolling.
Chuck (modeling Central Japan in September, 1964 - with helpers!)