A.F. Interlocking Tower (Site)

These coordinates mark the exact spot where the building once stood. No visible remains exist.


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Here follows an excerpt from "The Iron Road of Franconia" book about the railroad written by local historian Nathaniel Lee.

Some areas have more than one train track, with some tracks going in all different directions, places such as yards, crossings with other railroads, or high traffic areas. These were places where manned signals remained a necessity. The railroad company installed manned "interlocking towers" at these spots. Each tower operator was in charge of controlling all the switches by hand and telegraphing the train dispatcher when the train had passed their tower. A two-letter code distinguished each individual tower in a telegraph communication. The letters typically related to the name of the nearby town or station and prearranged so they could not be confused with other letter codes used in telegraph messages. For example, the tower located near the Telegraph Road overpass south of Alexandria controlled the trains running to and from Fredericksburg. The letters "AF," which stood for Alexandria to Fredericksburg, identified the tower.

Before those towers were prevalent along the railroad, a signalman would be required to physically set each signal and switch to the desired indication by hand. Under the old method of train dispatching, those people whose job it was to schedule and coordinate all the trains had to rely on reports from each individual signalman or telegraph operator to know the location of the trains out on their section of the track. The Alexandria and Fredericksburg Railway sometimes required trains to stop and collect written orders from a stationmaster in person. Not only was this an extremely slow process, it was also very expensive to the company's bottom line. It did not take railroad companies long to recognize that managing a series of signals from one central location would improve the effectiveness of workers and trains.

The first interlocking towers used levers. Simply raising or lowering those levers altered the warning given out by the signals, and changed the corresponding switch on the track. Usually, these levers had some type of prearranged color-coding to let the operator inside the tower know which warning the signal would display depending on which way the lever pointed. Interestingly, these hand-controlled levers stayed in use all the way through the end of the twenty-first century in some locations. The interlocking mechanism itself was in fact an early type of computer. The creative design of the interlocking machine permits the tower operator to control a group of levers, setting train routes in advance, without actually allowing a route that would cause an accident. Once a train entered a block, the interlocking machine locked in the set of switches ahead of it, and the machine would not let another train pass through those blocks. A tower operator could order the train to stop in an emergency, but the train's course was set such that the operator could not alter it until the train had completely passed through the block. This "approach lock" especially protected high-speed trains passing through that could not stop quickly. The railroad company designed and built each interlock device exactly for the section of track it would control.

Another interesting feature of the railroad interlocking towers was in their construction. They installed the interlocking machine on site first, and then built the frame of the tower building around the interlock mechanism. They built interlocking towers as two-story buildings due to the immense size and intricacy of the machine parts. The machine normally was located on the first floor of the building, along with a furnace and storage spaces. An outside stairway lead to the second floor, where the operator and switches were located and it featured wide windows looking towards the tracks in every direction for optimal viewing.

There were two main interlocking towers controlling the tracks through Fairfax County. GS Block, later known as CW Tower, stood on the north side of Pohick Creek and remained in operation until 1949. The second tower was AF Tower in Alexandria, located near the Telegraph Road overpass. It operated between 1906 and 1921 in a wooden building, then fifty more years in a brick building on the same site. From the beginning, AF Tower had responsibility over a unique joining of the Southern Railway and the Washington Southern Railway. The designation of "VM Crossing" as the telegraph call sign referred to the Virginia Midland Railway, the Southern Railway's predecessor railroad. The two railroad lines once crossed at-grade until the traffic of both railroads became too much to handle. The solution the two companies agreed upon remains in operation today. It features a track that parted from the north side of the Washington Southern tracks at Seminary Stop, just a half-mile west of AF Tower. Here, a long arching track brought the Washington Southern tracks onto a bridge above the tracks of the Southern Railway. There, the Washington Southern continued southwest, and the Southern Railway continued northwest.

As technology got better over time, railroad operators presided over tracks from long distances. Taking the place of the interlocking towers was a form of centralized traffic control from the RF&P Railroad headquarters in Richmond. Under the new centralized system, a single train dispatcher in Richmond now had complete control of the switches and signals over the entire length of the track. Control boards provided their operators a scale model picture of the track they managed and what switch points they controlled. This was a large console with a series of lines depicting tracks, switches and other miscellaneous track structures for the entire length of the railroad line. Electronic information concerning the progress of a train transmits to the control office instantly as a train passes through a section of track. At each track switch depicted on the console, there was a small light bulb and a small lever. When the light bulb turned off, that meant a train was occupying that block and if the train needed to transfer to another track, the operator sitting miles away could simply turn the lever or push a button. Instantly a signal sent to the relay box out on the track in turn operated a motor for the switch that put the train onto another track.

The new system saved trains an average of forty seconds of time for every mile on its route. Importantly, it permitted trains to make up time without delaying other trains by carefully scheduling the use of different tracks. Not only did the railroad's effectiveness improve, but a corresponding decline in expenses was also something the RF&P Railroad management found irresistible. No longer needing the manned signal towers with this amazing new technology, the railroad began to demolish these structures. By early 1971, the RF&P Railroad had become the nation's first railroad to have centralized electronic traffic control for every mile of track in its system.