The Federal rules covering engineer licensing and certification specifiy that speeding above a certain level over the authorized speed is grounds for mandatory decertification of the engineer. IOW, he can lose his license, which often results in loss of his job.
Therefore, it only makes sense to have a speed indicating device, and the new FRA Part 230 requires a speed indicating device on locomotives that oeprate on the general system and/or operate at more than 20 mph. There is NO requirement for a speed RECORDING device on steam locomotives.