Glad you found what seems like the bulk of the problem Don!
With a constant voltage set, does the train slow down and/or stall in the same place every time? What happens when you run on the same loop in the opposite direction (i.e., clockwise instead of counter-clockwise)? If you connect an ammeter in series with the power feed from the transformer, what does it show as the train begins to slow down and stall?
I like the starting torque of the old AC motors when I run my trains hands-on. But series-wound universal motors are inherently sensitive to changes in load. If you're really determined to "set it and forget it" at 30 mph or less, you would probably be happier with modern, can-motored loco(s) that have speed control. Keep us posted!
