Re: Helper Engines Placement

Here's a few more thoughts on the subject.
The determining factor of when/where to cut in helpers has primarily to do with the trailing tonnage behind the head end power of a given train. As you can imagine, the strength of the drawbar/coupler on a car just behind the lead locomotive (as well as the coupler behind the tender itself) is designed to handle just so much weight in tow before its integrity is compromised.
This is further complicated when it comes to the older narrow gauge equipment's wooden frames as I would imagine the coupler strength on some of these cars could well exceed that of the integrity of the car itself. In talking with some old narrow gaugers of the late 50's and 60's, this often seemed to be the case in an age of deferred car maintenance. Placing helpers back in the train (exact location and amount of locomotives specified by special instructions) will therefore add the power needed for the train while not exceeding the maximum drawbar limit/stress on the head end, thus avoiding a break in-two. Each subdivision had (and still has) their own special instructions regarding this based upon the grade profile. The steeper the grade, the more continous stress/weight placed on the drawbar at maximum effort on a given amount of cars or trailing tonnage. Of course, as Mr. Scholl pointed out, other unique features of the road such as bridge restrictions, excessive curve radius combined with tonnage considerations, could often play a role in the company's helper placement rules.