Re: Helper engines

If you run 2 seperate trains you need 2 entire crews. (Engineer, fireman, conductor and 2 brakemen). You also have to dispatch 2 trains with 2 sets of orders and make sure they stay apart and don't get together where they aren't supposed to. You also need 2 cabooses.
With a helper all you need is an engine crew. Once you get to the point where you could pull the train with one engine, you've essentially doubled your cost without increasing revenue.
This is one reason why freights on the NG got rather irregular in the end - they waited until there were enough loads to warrant a goodly sized 2 engine train west out of Alamosa, before they'd send a train out again.