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Sunday, January 22, 2012

Great Northern Railway Signaling: Then and Now

The reason for this Then and Now post is because I recently completed a trip via Amtrak's Empire Builder that included a trip over the BNSF Senic Subdivision which includes, among other things, the famous Cascade Tunnel, longest in the United States. Along this trip I accounted for the "now" portion of the signaling by taking almost 400 photos from the rear of the train documenting almost the entire Subdivision except for a section around Everett where I had to go get dinner.

The "Then" portion of this post came up when I found a 4 part YouTube video series taken by a young boy whose father was a block operator for the Great Northern Railway in the period between 1957 and 1971. The 8mm home movies show mostly GN trains, but does pay a good deal of attention to signaling and train orders with some shots inside the Skykomish block station and a sound track that consists of dispatcher-to-train communications recorded on a reel-to-reel tape ,achine. The Skykomish office is shown at the very beginning of the second part and shows the 4-level table interlocking machine that controls the local siding as well as a CTC display board that at least reports train positions (a unit level panel was not shown).

What is today known as the Senic Subdivision exemplifies the model of a North American single track railroad. The Main Track is broken up by 1-2 mile long passing sidings, which today are controlled by CTC, but in that time were usually worked by the crews using hand throw switches and written Train Orders. The Cascade Tunnel was built in 1929 and until 1957 was worked by electric locomotives running in a special district between Skykomish and Wenatchee. Therefore the siding and yard at Skykomish were given the luxury of both power signaling and an on site train order station. Even after the electrification was replaced by a ventilation system for diesels, the Skykomish block station remained in service as it still had local and CTC control duties and the yard was still needed as a helper base. Also, the operator at Skykomish had the important task of controlling the ventilation fans in the tunnel.

Here are the links to the 4 videos (remember part 2 starts off with both "tower" footage and recorded "tower" audio).

My 2011 photos can be found here: ... nails.html

The photos covering the parts most frequently seen in the videos begin here and you can just keep clicking next. One interesting bit of signaling I wanted to cover was that of the Cascade Tunnel itself. The tunnel is 8 miles long, but since only one train can navigate the tunnel at a time due to ventilation constraints it functions as one long absolute block. Speed is only 25 mph so that combined with the need to vent the tunnel for up to 20 minutes after an eastbound passage makes the limiting capacity constraint on the line. The interesting bit is that to keep the air blowing in the right direction a door is closed on the fan plant end until just before the train exits the tunnel. The door is unable to be interlocked with the signaling system because it must open so close to when the train exits that there is might not be enough room for the train to stop should it fail to do so. For this reason a spare door is on hand in case the main door were to suffer a catastrophic failure. Such a failure has only happened once since 1957.

Here is a video taken from the rear of the Empire Builder in the final 1/4 mile of its transit of the tunnel as it passes through the fan plant and then out past the westbound absolute signal. The door opens about at the point where the video begins.

And here is another video showing the amount of time between the door opening and a train passing it (40 seconds).



  1. I need to be wary of how much I say being an employee, but I think I can say the tunnel door is actually tied into the signal system. The door itself is protected by absolutes. Inside the tunnel is two intermediate signals. One for WB trains to advise of the signal at East Scenic, and one for EB trains to advise of the door signal.

    Another thing of interest, just prior to the west portal is a signal with two lunar aspects. They both flash for WWD trains to indicate the fans are operating.

    This is a neat blog too! I liked the piece on Caltrains (how I stumbled across this page...) and the change to their current signal system.

    1. I am sure it is protected by signals, but the time between when the door opens and when the train emerges is not sufficient for all trains to be able to stop in time. The door opens when the train is around 1/4 mile away.

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