Interlocking tower
From Wikipedia, the free encyclopedia
 |
It has been suggested that this article or section be merged into Signal_box. (Discuss) |
 |
This article does not cite any references or sources. (April 2008) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed. |
An interlocking tower is the name given to an often towerlike structure or building housing the control equipment for a railroad interlocking in North America. In other countries this is often referred to as a signal box or signal cabin. Once a critical part of the railroad infrastructure, with the advent of computer and telecommunications technology, as of 2007 there are fewer than 120 active interlocking towers in North America. These towers were, and commonly still are, known as switch towers or signal towers.
[edit] History
Soon after the advent of railroad technology, increasing traffic levels made finding a way of safely routing trains through junctions with multiple conflicting routes a necessity. The concept of a railroad interlocking involved having a junction or other segment of track placed under the control of an interlocking machine. This machine would prevent switches to be aligned or signals to be displayed in an unsafe manner. The early interlocking machines were purely mechanical in nature with manually operated levers moved the switches and signals and a complex network of interlocking bars and cams provided the protection. Interlocking machines were large and complicated devices and a two story "tower" was the natural solution to house them. The upper story provided a place for the block operator to work with superior visibility of the interlocking plant and the lower story would house the actual interlocking mechanism as well as the connection to the pipelines which drove the switches and signals.
As time progressed the mechanical interlockings were augmented with electric signals, track circuits and electro-pneumatic, electromechanical, straight electric and relay based interlocking machines were installed in towers. The lower level went from housing the mechanical lever frame to housing the relays, transformers and air compressors used by the more modern systems. The newer technology allowed for small towers to be consolidated into larger ones, with reduced staffing requirements. Centralized traffic control (CTC) systems began to be placed in towers starting in the 1930s allowing a single tower to control large segments of track.
The advent of Centralized traffic control (CTC) would eventually spell the doom of the interlocking tower in North America. Towers had always operated under the control of a railroad dispatcher who would be in charge of directing traffic over large parts of the system. With interlocking towers railroads would use a system called paper dispatching where a dispatcher in a central office would communicate with towers and keep track of the locations of trains on a large paper ledger. Most orders regarding non-scheduled movements would need to come from the dispatcher. As CTC technology got better it became easier for the dispatcher to control trains directly on his own CTC panel or computer terminal and as a result towers were largely eliminated.
In the 1950s and 60's, railroads like the New York Central, Canadian National and Canadian Pacific were very aggressive in converting to CTC. As computer technology matured in the 1980s CTC adoption became widespread and by the early 1990s most Interlocking Towers in North America had been closed, with the exception of about 250 interlockings that were very busy, involved draw bridges, involved crossings at grade or which were just forgotten about by management. After the year 2000 railroads have become more aggressive about eliminating interlocking towers as their aging interlocking machines have become harder and harder to maintain. Still, a few lines still see the value of towers in places with very high usage.
