49 CFR §229.117

1. (a)After December 31, 1980, each locomotive used as a controlling locomotive at speeds in excess of 20 miles per hour shall be equipped with a speed indicator which is—
   1. (1)Accurate within ±3 miles per hour of actual speed at speeds of 10 to 30 miles per hour and accurate within ±5 miles per hour at speeds above 30 miles per hour; and
   2. (2)Clearly readable from the engineer's normal position under all light conditions.
2. (b)Each speed indicator required shall be tested as soon as possible after departure by means of speed test sections or equivalent procedures.