The rolling resistance for steel on steel is quite low; the coefficient of rolling friction is typically μr=0.002. suppose a 180,000 kg locomotive is rolling at 12 m/s on level rails. if the engineer disengages the engine, how much time will it take the locomotive to coast to a stop?

M =  180,000 kg, the mass of the locomotive.

The normal reaction is
N = mg
   = (9.8 m/s²)*(180,000 kg)
   = 1764,000 N

The frictional resistance is
F = μN
  = 0.002*1764,000
  = 3528 N

The deceleration, a, caused by the frictional force is
a = (3528 N)/(180,000 kg)
   = 0.0196 m/s²

The initial velocity of the train is 12 m/s.
The time, t, required for the train to coast to a stop is
(12 m/s) - (0.0196 m/s²)*(t s) = 0
t = 612.24 s

612.2 s