Central Venue Interlocking Part 2

Each day a variety of passenger and freight trains passed through the Central Avenue area. This view shows Great Northern Train 19 leaving the depot on its way toward the tower to St. Paul in August 1965. Tower operators needed to know in advance which trains were approaching in order to be able to line up the tracks and signals to get each train onto the correct route for its ultimate destination. Central Avenue used a bell system that alerted the operators to approaching trains. Each route had its own ring tone much like Morse code. (Image: Marvin Nielsen)

Last month we explored the history and executive battles that surrounded the railroad territory at Central Avenue in South Superior, Wisconsin. This month’s story explains how trains were safely routed through the area.

The Dispatcher’s Hidden Helpers
The men who kept track of all of the trains moving across a division on a railroad were the dispatchers. Each of them worked an eight-hour shift, or “trick.” These so-called trick dispatchers then reported to the chief dispatcher who was known to all as the “chief.” The chief depended on his team of dispatchers to keep order over all the trains operating on their division, and consequently they depended on the chief to help get them out of a jam when problems arose. Everyone in the dispatcher’s office depended on the train crews. It was their responsibility to keep each train rolling across the division. Everyone did their best to stay on schedule, but that could prove quite difficult without the help of a third group of “hidden” railroaders who were absolutely essential to the entire operation: the “Tower Men.” These railroad workers manned the control towers located at busy railroad crossings and junctions. Each was in effect the “traffic cop” who signaled to the trains which of them could proceed through the railroad intersection and which of them had to wait their turn at these busy junction points. They also provided the muscle that physically moved the levers that aligned the track switches and changed the signals from red to green.

The interlocking towers were designed so that the operators or tower men could see out and over the top of each passing train. If a load was shifted or if brakes were smoking badly the operator could see, hear, or even smell the problem. The train crew could then be notified by radio to stop the train. The men inside the towers provided an extra measure of safety by visually inspecting every passing train. The last stand for the old “covered wagons” on Burlington Northern was making freight runs between Superior and either Minneapolis or Staples. This early morning shot in July 1982 shows Train 127 with ex-Northern Pacific EMD F9As 770, 792, 790, 774, and 794 on the point rumbling southbound past the tower on interlocking route C-D. (Image: Don Larson)

The Interlocker
Technically speaking an interlocking complex (or interlocking plant) is a series of mechanical devices that must be operated in a predetermined sequential order to line up track switches and signals to allow one or more trains to safely pass through the interlocked area of tracks. This is precisely what an interlocking does—on any railroad—whether it is mechanically or electrically operated, locally manned or remotely controlled.

Because Central Avenue had so many tracks and connecting lines, a 40-lever Union Switch and Signal Company interlocker was required. This was a large, complex machine that weighed several tons. The interlocker was contained inside of a tall, 2-story, wooden structure known as the interlocking tower. It’s the tower in which the tower men worked and it really was the heart of Central Avenue on the railroad. In fact, many early communities depended on these kinds of towers to serve as temporary depots for freight and passengers. Some were even used as a community’s first city hall because they were often the first permanent structures with heat, light, and a communication system.

Burlington Northern 1364 on the Carlton Local pulls its 13 cars and caboose through interlocking route D-B on its way back to Superior on May 10, 1975. Notice the rods that parallel the track. These are the rods that are connected to the levers inside the tower. When the tower men threw the levers, that linear motion was transferred into these rods that were connected to track switches, switch point locks, and signals. But just imagine this same scene in the dead of winter with 4-5 feet of snow on the ground. It took a lot of work to keep these rods lubricated in warm weather and clear of snow and ice during the long winter. (Image: Robert C. Anderson)

In actual operation, the interlocker was a mechanical device that operated track switches and signals that allowed trains to move safely through the plant without colliding. These track switches and signals were moved mechanically by hand-throwing long steel levers located within the tower’s top floor. When the levers were thrown, that mechanical motion was transferred outside of the tower and sent along dozens of steel rods mounted on rollers in close proximity to the ground. The rods were hollow to keep them as light as possible. Some rods even traveled underneath the tracks to get to distant parts of the plant. Bell cranks were used when the linear motion needed to go left, right, up or down. Some of the switches being thrown were hundreds of feet from the tower. A large man with some weight on him could do the job fairly easily. But it did require significant strength to throw the purely mechanical levers. Many tower men called them “Armstrong” levers because you needed strong arms to do it. This was especially true in winter months when ice and snow would build up on potentially thousands of linear feet of steel piping.

This Northern Pacific map of Central Avenue interlocking shows the facility as it existed in 1932. This plan served the purposes of the signal department so that they understood how the signals should operate when a train occupied any given route through this area. This plan wasn’t used by the tower operators though. They needed something more specific that would guide them through the process of throwing levers to route trains. (Image: Hudson Leighton, Twin Ports Rail History Collection)

As the tower building was a manned control point, it allowed the operator to route trains properly. It also simultaneously prevented accidental collisions. Here’s how. Working high above ground level the operator could easily see what was coming down the tracks through any of the many windows that surrounded his working stage. The levers were arranged in numerical order and each corresponded to a particular switch and signal along the tracks. The more routes that were available through the interlocking, the more different combinations of levers would need to be thrown to create those different routes. Each tower had a large chart on the wall above the levers so that it was easy to determine the route. Each possible route was assigned a multi-letter combination (e.g., A-B, C-D, or J-F, etc.) known only to the tower man. A series of numbers assigned to that letter combination then determined exactly which levers needed to be thrown and the exact order in which to throw them. Simple routes required the throwing of just two or three levers. Most were five or six. The most-complicated routes required 10. Tower men had many duties but if they were generally fit and could match numbers on a chart with numbers on the levers, then they could certainly handle this particular part of the job.

Click on “2” for the rest of the story….


  • Lemke, Jeff. Twin Ports Rail History. Zenith City Press, Duluth, Minnesota: forthcoming.
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