Frank,
This discussion came up a year or so ago on another board. I mentioned that I had heard of the 1.1 rule and John Craft sent me some better information on the subject. Basically, there is an empirically derived upper limit of 540 rpm, above which the drivers start to lift off the rails (when centrifugal force of the counterbalance exceeds the weight on drivers), the valve gear parts start to bend (due to the inertia of the piston and valves), etc.
A few years ago, a British Pacific, "Blue Peter," in the hands of an inexperienced engine driver, went into a wild, uncontrolled wheelslip and the valve gear wound up bending like a pretzel. From video footage, they were able to calculate that the drivers were turning at what would have been 136 mph when it flew apart. The 540 rpm rule yields a top speed of 129 mph for that engine.
If you know the driver diameter, you can calculate this "theoretical" maximum speed. Lots of engines could not have reached this speed for other reasons, such as counterbalancing, lateral motion control, track bed, etc.
The K's with their outside cranks and the thrust axis of the cylinders so far outside the track gauge, are probably never going to come close to the 540 rule speed. The T-12's, with inside frames, light rods and four wheel lead trucks were another story. Speeds in the 60 mph range have been rumoured for them. Marty's 30 mph placard is just a twitch under 1/2 the theoretical limit for a K-27. Having driven alongside a K-37 north of Durango and watched it bob and weave like a boxer, I'd say 30 mph would be plenty fast for any K!
