To all of our surprise my problem seems to be center rail pickup.
I sanded the black off the idler wheels and attached hot and neutral jumpers.
The neutral jumper had not effect (although cleaning the wheels may have helped) and I was still stalling, but the hot jumper resulted in perfect performance!!! Crawled through my double slip at 1smph repeatedly!!
I'm going to remove the neutral jumper and attach the hot jumper to a roller on a caboose and do some more testing, but I'm definitely happy to have a preliminary diagnosis.
I have one idea of how to add rollers. I'll try to post a sketch in the morning.
Anyone have ideas?