Other than the condition of the track (plus curves, grades and location of signals), limiting factors are boiler capacity, driver diameter, counterbalancing, main rod/piston thrust, and side rods. Maybe the valve stem and its valve in the steam chest if they move back and forth too fast. Exceed maximum safe speed and something will give way.
Generally, steam locomotives designed for high speeds have tall (big) drivers and 4-wheel pilot trucks to guide them through curves. Their frames and equalizing systems [allow drivers and trucks to move up and down and side to side] must be up to the task, too.
Nickel Plate Berkshires excelled in fast freight service with 2-wheel pilot trucks.
A rough estimate is 10 mph for every inch of driving wheel diameter.
Shays and other geared locomotives are different because their cylinders drive a telescoping shaft that turns the wheels via gears, not rods. Shays sound as though they are going 100 mph as soon as they exceed walking speed. The best place to hear that is Cass Scenic Railroad. Past the enginehouse, when a Shay begins to push a train upgrade, the engineer widens the throttle and the exhaust from 3 cylinders erupts into a roar.
Exhausts on Heislers and Climaxes are a bit slower because each has only two cylinders turning a driveshaft centered below the boiler.
I suppose a geared locomotive, pushed beyond its limits (15 mph - 20 mph), would suffer a failure in the drive train.
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