Another factor in how fast a locomotive will run is in how the valve gear is set up. Locomotives built to run fast are set up with longer lead in the valves. Lead puts steam into the cylinders before the piston gets to the end of the stroke, giving a little extra compression and push to the piston when main steam admission starts - along the same lines as "ignition advance" in a gas engine. Too much lead makes it difficult to start a heavy train and tends to make the engine pound when working hard (just like "spark knock or pinging"). Stephenson Valve gear has an advantage here it that it has a variable lead. The higher you hook up the gear, the more lead you get, the faster you can go. Walschert Valve gear has a set lead. It's set in the shop and you always get the same amount, no matter where the gear is set. The designers of the locomotive design the gear for the service the locomotive is designed to perform. Narrow gauge power is not designed to go very fast and subsequently has very little lead. While it was common for passenger engines to have 3/16"+ of lead, the DRGW was very conservative with the NG power and specified only 1/16". Jack and I thought that was awful small, and it would take vary little wear in the valve gear to loose that 1/16", so when he set up the 463, I belive he opened up the lead to 1/8". It never failed to get a dead ass heavy train started, and when hooked up, she'd run real well. 489 used to do the same and often wondered if the Salida boys who did the last DRGW shopping on it didn't think the same way.
I think the old 1.1" per MPH deal is a model RR thing. I remember reading that in Model Railroader years ago.
So my not so scientific analysis is a T12 with 46" drivers, a nice long wheelbase, a good 4 wheel pony truck, light rods, good Stephenson Gear set give ample lead when hooked up tight, could run about as fast as you wanted as longs you had two things: 1. ample steam or 2. plenty of guts.
A K28 might be faster than a K36/37 because is lighter rod assemblies, but eventually the limiting factor will be steam admission to the cylinders.
