ED,
I was curious so I took a Atlas F3 with twin vertical mount motors and hooked it to my 6 GGD columbian cars and then added two more GGD aluminum from my Cinncinatian. The single A unit had no problem pulling the 8 cars up 1.5% grade which wraps around a 180 degree curve. I added 3 more cars but the single A unit slipped with the 11 cars. If I added the dual powered 2nd A unit, it took off with all 11. This is a 2010 version Atla F unit, not the earlier version from the 70s.