49 CFR 230.105: Lateral motion.

(a) Condemning limits. The total lateral motion or play between the hubs of the wheels and the boxes on any pair of wheels shall not exceed the following limits:

Inches

Engine truck wheels (with swing centers) 1

Engine truck wheels (with rigid centers) 11⁄2

Trailing truck wheels 1

Driving wheels 3⁄4

(b) Limits increased. These limits may be increased on steam locomotives operating on track where the curvature exceeds 20 degrees when it can be shown that conditions require additional lateral motion.

(c) Non-interference with other parts. The lateral motion shall in all cases be kept within such limits that the driving wheels, rods, or crank pins will not interfere with other parts of the steam locomotive.