Since 1 hour is 3600 seconds,

 1 second is 1/3600th of an hour, so
 6 seconds is 6/3600ths of an hour of 1/600th of an hour
30 seconds is 30/3600ths of an hour of 1/120th of an hour

Let the train's length be x mi.
Let the tunnel's length by y mi.

When the front of the train is at the tunnel entrance,
the rear of the train is x mi. from the tunnel entrance.
It then takes the rear of the train 1/600th of an hour
to get to the entrance of the tunnel.  In that time the 
rear of the train has traveled exactly one train-length 
or x mi (to get to the tunnel entrance).

DISTANCE = RATE·TIME so

       x = 60· =  =  of a mile.

So the train is  of a mile long. (which is 528 feet)

It then takes 1/120th of an hour for the rear of the train to
get from the tunnel entrance to the tunnel exit.  In
that time the rear of the train has traveled exactly one 
tunnel-length or y mi.

DISTANCE = RATE·TIME so

       y = 60· =  =  of a mile.

So the tunnel is  of a mile long. 

Edwin