When deciding were to put helper engines in a train, you have to look at train make up as well as the grades and curves to be encountered.
In the case of the D&RGW, having wooden cars on the head of a long train climbing a steep grade could exceed the drawbar strength of the lead cars, causing the lead cars to get a drawbar pulled out, or of the cars were empty or lightly loaded, cause the cars to derail on the inside of a sharp curve. To overcome this problem, the helper would be placed back in the train. Putting the helper on the rear ahead of the caboose solved that problem, however, if you had a bunch of empty wood cars ahead of the helper, you could still have problems and fold up weaker cars ahead of the helper, in which case, they would position the helper mid-train so that the trailing cars behind the helper represented the cars the helper was pulling.
Even in modern times, this can be a problem. When I worked on the standard gauge out of Alamosa over La Veta Pass, we were restricted in tonnage that could be behind the engines. I forgot the number, but it worked out to about 33 loaded cars. Any more than that, and helper had to be cut into the train or put on the rear.
Putting helpers in a train in rolling up and down country, required them to be on the head of the train, as the heavy helper engine in the middle or rear of the train could cause some serious slack action and pull the train apart. This is why trains between Chama and Durango were doubleheaded.
One other big factor in placing helpers was bridge restrictions which would not allow trains to be doubleheaded, in which case you put the helper back in the train, or stopped at the bridge and ran each engine across separately.
To comment on OC MP62's post about using sand going down hill, yes, that works well, and I have had to resort to that many times. Some times if I had a poor handling train, I laid more sand coming down the hill than I did pulling it up!
