Let length of tunnel is x meter
Distance = 800+x meter
Time = 1 minute = 60 seconds
Speed = 78 km/hr = 78 × 5/18 m/s = 65/3 m/s
Distance = Speed × Time
=>800+x=65/3 × 60
=>800+x=20 × 65=1300
=>x=1300-800=500
So the length of the tunnel is 500 meters.
[If you want to calculate it practically, then please have your copy and pen
and take a ride of Kalka to Shimla toy train in north India. It has many tunnels on the way :) ]