There are several issues involved. Larger locomotives like the D&RGW K engines could handle pretty long trains with only two engines.
In some cases, the combined weight of locomotives over certain bridges could be a problem, as at Lobato, where the lead helper must cross light.
In other cases, it might involve the consist, and in some cases, concern over drawbar loads on long trains, especially with older wood frame cars. Weather could also be a factor.
On the C&S, with smaller engines, multiple helpers were very common on freight trains. In most cases, there would be two engines doubleheaded on the front, with one engine cut in ahead of the caboose, and on larger trains a third helper cut in about 5 cars ahead. This would be done largely to reduce drawbar stress, but also for communication between the engines, as visual contact with each other as well as the train maximized, and whistle signals could be more easily heard and relayed.
So, a lot of varied reasons to consider.
Edited 2 time(s). Last edit at 11/30/2017 04:58PM by Mike Trent.