I handled a few cuts of 50, 60, possibly 70 cars with an RS-1 back in 1981, re-organizing a storage yard at a chemical plant. The locomotive was ex G&W 43 (C&WI 256). Admittedly we didn't go very fast; didn't dare, track conditions, but a ten car train over most shortlines should be well within the capabilities of a 1000 HP Alco.
When we were up at G&W picking out the 43, we saw two of the other RS-1, with MU, take a 40-some car train out of Retsof. By the time the last car came past, the two units had the train up to 25 or 30. Pretty decent little units, unfortunately, they were much younger then.