Ben, I can tell you this... prior to 112's repower, she would be in full throttle with just four cars, and it was still a struggle.
I know this doesn't apply to the A&A, but running helpers on Horseshoe Curve, I learned that rear-end helpers are much more than just shoving. One thing a good rear-end crew will do is help keep the train stretched or bunched. Keeping the slack from free-rolling through the train infact burns less fuel/brakeshoes.
In this case, it might have just been a case of better safe than sorry. Since it has been fifteen-plus years since 18 was at Arcade Junction, it makes sense that the crew wouldn't know how she would handle with that type of tonnage and equipment, especially on the grades involved.
If memory serves (bear in mind, I was just a conductor trainee then, not learning train handling), its down grade from the junction to the crossing behind Yansick's. Then upgrade all the way to West Main, with it increasing just west of the crossing. Plus, there you have the curve as well. I don't remember the grade from West Main to Park, but I think its relatively flat. Then you have the really tight curve around the wye.
With the grade and curve combinations, combined with the unequal torque produced by a standard reciprocating steam engine and add in the train tonnage, it would be likely that 18 would have problems in certain spots, even with dry rail and sand. After the torturous endevour to get her back into service, I can only imagine the heartbreak
Figuring cars that are fully loaded at 286,000 pounds, a six car train would tally 858 tons. But, don't forget, 18 also has to deal with the tonnage of her tender, so lets say just about 900tons, (1.8 million pounds).
And, while I can't speak for the loaders & shippers, it isn't unheard of for a car to be overloaded, or under-reported.
Finally, 111 could simply have been along to provide a shove to keep the cars moving through curves where the risk of straight-lining exists.
I guess it boils down to: you'd have to ask Brad.
__ J. D. Gallaway __
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