On Operation
Tower mysteries revealed
The word “mysterious” well describes interlocking towers, which have long fascinated me. Rising high above the rails, second-story windows darkened by awnings or shades, they suggested guards posted in observation points along a main line. Friendly waves often came my way from engines and cabooses, but rarely, if ever, from a tower. Nevertheless, I sensed importance in their placement at key junctions.
Curiosity finally had me climbing the stairs of Erie Lackawanna’s tower at Newburgh Junction. Its remote location appealed; I reasoned that towers in lightly populated areas were less likely to greet intruders with a boot in the seat of the pants. I eased the door open and let myself in, expecting the worst. Instead, I found a welcoming operator, happy for some company.
Train and engine movements at busy places such as terminals, yards, and junctions were made more practical if a single location controlled the switches and signals governing them. The more complex, the more likely an error by an operator, so interlockings required numbered switches and signals to be operated in certain order to prevent collisions and derailments.
The first interlocking machines were entirely mechanical, relying on sliding bars and dogs which interfered with one another. This interlocking action kept the operator from mistakenly creating a conflicting movement by using the wrong lever. Like the sequence required of a combination lock, only when all the switch levers were in proper position for a given route could he operate a particular signal lever and clear a signal for movement through the plant. Electrical circuits, relays, and today’s microchips replaced the mechanical devices as technology evolved. Dave Abeles’ new Kalmbach book, Guide to Signals and Interlockings, goes into detail.
Timetables, lineups, and communication with the dispatcher and other operators helped an operator, sometimes called a towerman, plan his actions on the machine. Annunciator bells also alerted him to a train’s distant approach. I remember how Mike Bednar’s smile lit up R Tower in Allentown, Penn., as he swiveled in his chair and answered my greeting with “The Star just hit the bell at Burn.”
The operator observed movements on a model board, typically mounted atop the machine. It was a line diagram of the plant’s tracks and signals on which track occupancy lights and other illuminated indicators were displayed. Letters designated entrances and exits from the plant. Manipulation charts, one in each direction, were also displayed. The charts gave the lever numbers required for routing between any given entrance and exit, for example, “A to E 18 26 7.”
And towers came in all sizes. Fourth Street Tower at Southern Pacific’s Third & Townsend terminal in San Francisco preceded air traffic control tower designs, its top floor capping two thinwaisted stories like a mushroom. At the other end of the spectrum is the nondescript shanty in the photo above, simply adorned “Kentucky St.” But its size belies its importance. Look closely and you’ll see train order forks at the ready, an out-of-service order board mast, and a tangle of communication and signal wires. Kentucky Street funneled Missouri Pacific, Frisco, Rock Island, and Cotton Belt movements over the two Mississippi River bridges into Memphis.
A quick web search for “interlocking tower” can turn up enough reading to make a rainy day fly by. Two sites
I like are Carsten Lundsten’s (lundsten.dk/us_signaling/) and Jon Roma’s (jonroma.net/ towers/). Carsten illustrates how movements proceed through plants and Jon has posted a comprehensive amount of prototype material. You’ll find enough information to inspire an interlocking plant for your layout that suits any budget.
THE FIRST INTERLOCKING MACHINES WERE ENTIRELY MECHANICAL, RELYING ON SLIDING BARS AND DOGS WHICH INTERFERED WITH ONE ANOTHER.
– JERRY