What I should have added is that vmax is in m/s… For km/h you would get:
m = P / ( 0.055 * vmax)
Also since those early steamers don’t have a lot of power, I use
m = P/ (0.025 * vmax)
to get a decent train length for those. But they will have trouble will with hilly parts, so best to keep them running in nice flat terrain!