The missing variable is the ratio of the linkage from the piston to the drivers...
That's the stroke. Think of a steam locomotive (most of them anyway) as a vehicle with the wheels mounted on the ends of the engine crankshaft. ATSF 3463 has a 29-1/2" stroke, which compared to other large steam locomotives could be called "moderate".
I did some comparison calculations just to see how this locomotive compares to some other fast steam locomotives. One famous high speed run was made by a Norfolk & Western Railway "J" class 4-8-4 doing testing on the Pennsylvania Railroad back in the 1940's. Now a J only had 70 inch tall drivers (maybe equivalent to a 3.54 auto rear end gear) but it managed to achieve 110 MPH. It also had a longer 32 inch stroke. 110 MPH wasn't considered all that fast in general, but it was very fast for a locomotive with relatively small driving wheels, which put its horsepower peak at a lower speed (say 45 MPH or so) which was good for the moutainous territory it operated in. One of the design features which enabled it to safely operate at high RPM was the use of Timken high-strength, light-weight main rods, connecting rods, piston rods, pistons and cross heads. The N&W J steam locomotive was turning 528 RPM at 110 MPH.
For comparison, the ATSF 3463 with its 84 inch drivers would be turning only 520 RPM at 130 MPH, and it would have a significantly lower piston speed because of the shorter stroke. The information on the website states they will also equip it with alloy rods and reciprocating parts.