Steam Boston-worcester Csx Q436 Selkirk-framingham - Possible In Anything But Ac4400cw?

I keep trying to do this service on the GP38, C40-8W, or SD40 but no matter how carefully I set the train up, it won't budge unless I use the AC4400CW. Is there some step I am missing that's needed for this train but not the other freight trains I'm used to running on other routes with these locos substituted as the lead unit?