Arailway train 50 m long passes over a bridge 250 m long at the rate of 12 km/h. how long will it take completely pass over the bridge?

From the instant when the nose of the locomotive reaches the beginning of the
bridge, it has to travel the whole 250m to get to the other end of the bridge, and
then it has to travel another 50m to get the rest of the train off the bridge.  So it
has to travel 300m to get everything it's pulling across the bridge.

Time = distance/speed = 300m / (12,000 m/hr) = 0.025 hour = 1 minute 30 sec

Pretty slow that train ;)

t=s/v
v=12 km/h=10/3 m/s
s=250+50=300 m

t=300/(10/3)
t=300*3/10
t=30*3
t=90 s=1.5 min