Interlocking tower

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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 Commonwealth countries this is often referred to as a signal box. Once a critical part of the Railroad infrastructure, with the advent of computer and telecommunications technology, as of 2007 there are less than 120 active interlocking towers in North America. These towers were, and commonly still are, known as switch towers or signal towers.

Contents 1 History 2 Structure 3 Operation 4 External Links

[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 1930's 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 1950's 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 1980's CTC adoption became widespread and by the early 1990's most Interlocking Towers in North America had been closed, with the exception of about 250 intelockings 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 demanding traffic loads or unique situations. The Long Island Rail Road has decided to keep its network of manned interlocking towers while upgrading the interlocking hardware and installing the capability for remote centralized supervision. The PATH also operates with 100% tower control, after having tried CTC and rejecting it. They opened perhaps the newest interlocking tower in the united States at the rebuilt World Trade Center Station in 2003.

In contrast to the 100 or so surviving interlocking tower, modern U.S. railroad dispatching centers supervise up to several thousand miles of railroad each, the largest being Union Pacific's Harriman Dispatch Center in Omaha, Nebraska, which controls over 30,000 miles of railroad.

[edit] Structure

[edit] Operation

Shifts

Interlocking towers are operated with a maximum of three 8 hours shifts per day Tower personnel in the United States are limited by the FRA to service no more than 10 hours before having at least an 8 hour rest. Most towers are operated 24 hours a day, 7 days a week as without a person on duty trains would not be able to move through the interlocking plant. However some towers do operate on a part-time or as needed basis. While operations some towers to shut down for the night, others can be equipped with an automatic operation feature where the levers can be set to a certain routing and then the tower put into automatic. The signals will function as Automatic block signals until the tower is switched back in. Other towers may have control transfered to another tower or dispatcher while it is closed.

As-needed towers frequently occur on lines where traffic is very infrequently (usually then at a movable bridge) or where the operation of the interlocking is only needed to accommodate track work or similar service disruptions. Some towers, like HAROLD in Sunnyside, New York have been supplanted by a CTC control center, but can be activated for various contingency plans.

Personnel

• Block Operator: In most single person towers this was the title given to the person on duty. A Block Operator is generally responsible for routing scheduled trains properly, reporting train movements to the dispatcher, following the dispatcher's instructions, reporting train movements to nearby towers (if any), passing up train orders and maintaining safe operations regarding out of service portions of track and/or men at work on the line.

• Train Director: In busy towers with multiple staff a Train Director was often in charge of one or more levermen who would be responsible for working the interlocking machine. A Train director would generally have the same responsibilities as a block operator, but receive extra pay and would sometimes have more routing discretion. Sometimes a single person tower is staffed with a Train Director rather than a block Operator as the duties of that tower may warrant a higher pay grade.

• Leverman: See above.

• Telegrapher: Some very busy towers would have a telegrapher position who was responsible for all of the various paperwork and train reporting duties. The name stuck ever after the telegraph was abandoned for voice communications via telephone. Amtrak's DOCK tower in Newark, New Jersey still has a telegrapher job for the morning and afternoon shifts.

• Maintainer: Most towers would have their own signal maintainer assigned to them. The maintainer would sometimes have a little workshop in the basement or relay room where they could store spare parts and fix failed components and would also use the tower as a place to sign on duty and store equipment, personal items and park maintainence vehicles. Make closed towers are still used as maintainence bases.

[edit] External Links

List of active and recently closed North American interlocking towers