The Cascade Tunnel on the Burlington Northern Santa Fe line between Seattle and Wenatchee Washington was built in 1929 for electric operation. The tunnel is 41,152 feet (7.79 miles) long on a continuous 1.6% ascending eastward grade. The line was electrified when the tunnel was built. In the 1950s the line was converted to diesel locomotives and a means of forced ventilation was required. The ventilation provides combustion air for the locomotives as well as the breathing air for the crewmembers and passengers that usually comes to mind first.
The ventilating plant is on the east end (higher end) of the tunnel. There are two fans facing into a shaft that face into a shaft that joins the tunnel diagonally just inside of the east portal. There are two reasons for the door. First, the path of least resistance for the air blowing into the tunnel through the fans at the east end would be immediately out the door at the east end instead of through the length of the tunnel. Second, The tunnel is pressurized against eastward trains to ensure a good supply of combustion/cooling air for the locomotives and to limit the effect of the forward motion of the train on the smoke behind the locomotives. The forward motion of the train would pull the trailing smoke, keeping it near the head end of the train.
The door must be closed and one fan must be operating before an eastward train enters the tunnel.
When the door opens, the air pressure in the tunnel equalizes with the external air pressure. Some smoke can be seen exiting the east portal of the tunnel with the train, drawn out by the air currents generated by the train. After an eastward train leaves the tunnel, the tunnel must be cleared of smoke for 30 minutes before another eastward train may enter the tunnel. This 30 minute period is mistakenly considered the capacity limitation of the line.
There is no waiting period between the exit of an eastward train and the entrance of a westward train. The door closes and the fans begin operation as soon as the eastward train leaves the tunnel. In the time it takes for the eastward train to clear the west switch at Berne and the eastward train to reach the tunnel door, the east end of the tunnel is cleared sufficiently for the westward train to enter.
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