Index to pages

  1. Baltimore & Ohio
    1. East Norwood
  2. Bangor & Aroostock
    1. South Lagrange
  3. Boston & Maine
    1. Whitefield
  4. Central Railroad of New Jersey
    1. Plainfield
  5. Chesapeake & Ohio
    1. Scioto
  6. Chicago & North Western
    1. Lake Street
    2. Tower CK, Winona
    3. Wood Street
  7. Chicago & Western Indiana
    1. State Line
  8. Chicago, Burlington & Quincy
    1. Crawford
    2. Savanna (Ill)
  9. Chicago, North Shore & Milwaukee
    1. Dempster Street
  10. Chicago, Milwaukee, St Paul & Pacific
    1. Hoffman Avenue
    2. Savanna
  11. Chicago, Rock Island & Pacific
    1. Beverley Hills
    2. 45th Street
    3. 61st Street
  12. Chicago Transit Authority
    1. Tower 12
    2. Tower 18
    3. Wilson Avenue
  13. Delaware, Lackawana & Western
    1. Hoboken Terminal
  14. Great Northern
    1. Como Avenue
  15. Illinois Central
    1. Dubuque East
  16. Lake Shore & Michigan Southern
    1. La Salle Street Tower, Chicago
  17. Lehigh Valley
    1. Conway
  18. New York Central
    1. 16th Street, Chicago
    2. Grand Central Signal Station F
    3. Shelby
  19. New York Subway
    1. Chambers Street
  20. New York, Westchester & Boston
    1. Heathcote
  21. Northern Pacific
    1. Manitoba Junction
    2. St Anthony Park Junction
    3. Tenstrike
  22. North Shore
    1. South Upton
  23. Pennsylvania
    1. PRR Towers, Zoo to Nassau (MB)
    2. Beverley Junction
    3. Bryn Mawr (MB)
    4. Burnham
    5. County (MB)
    6. Elmora (MB)
    7. Fair (MB)
    8. Ford (MB)
    9. Grundy (MB)
    10. Holmes (MB)
    11. Hunter (MB)
    12. Lane (MB)
    13. Lincoln (MB)
    14. Midway (MB)
    15. Millham (MB)
    16. Morris (MB)
    17. Nassau (MB)
    18. North Philadelphia (MB)
    19. Shore (MB)
    20. Tuckahoe (MB)
    21. 21st Street, Chicago
    22. Union (MB)
    23. Winslow (MB)
    24. Zoo (MB)
  24. Union Pacific
    1. Argo (Washington)

Articles marked MB are written by Mike Brotzman

Although a little early British-style mechanical signalling equipment was found in the USA, this country forged its own route on signalling, pioneering power operation at an early date.

Signal boxes were nearly always (and still are) referred to as towers in the USA, and the lever frames are interlocking machines or interlocking plant. The New York Central particularly favoured the use of their two-letter telegraphic code identification of their towers - for instance PO for Porter, BO for Bottsford, and NE for Pine. When telephone suprseded the telegraph, some quite obscure names were adopted, such as Slope to correspond with SL, possibly becasue it was just easier to speak than reading the two letters out.

All mechanical (often called "strong-arm" or "armstrong") lever frames, or interlocking machines, that I know of were one of three similar models. Most were built by the Union Switch & Signal Company, which later to become part of the Westinghouse organisation. This was to an improved design of Saxby & Farmer's Rocker type (some had vertical beds of locking), created by a British-born engineer working for US&S in the 1870's. Rocker locking fell out of favour at an early date in the UK but was continued in manufacture much later in the US. The other popular type of frame was manufactured by the National Company, and looked broadly similar to the US&S type but lacked the rocker mechanisms, and had underfloor locking. A few Westinghouse type A frames were installed in later days.

Point rodding is known as pipeline, points are switches and traps derails.

Power operation of signalling was adopted from an early date, using all-electric Union Switch & Signal Co handle frames and electro-pneumatic slide power frames built by the General Railway Signal Co, of Rochester, New York. Both types of frame (extinct in the UK) can still be found in use today.

Block systems

Nowadays, most signalling is managed remotely using Centralised Traffic Control, a term first introduced in the United States but adopted (often with a different meaning) by many other countries. The operational definition of CTC has come to refer more to the Traffic Control of the name than the Centralised part. CTC indicates that a line has bidirectional operation on all of its tracks with remote-controlled power-operated crossovers and dispatcher control of the traffic direction on any track. Dispatchers can be up to 1,000 miles from the trains they are dealing with, so the major railroads have constructed microwave towers along their main lines to supplement communication by railway-owned cables and leased phone lines. Most of the BNSF system, for example, stretching as far as Chicago and the West Coast from Los Angeles to Seattle, is dispatched from Fort Worth TX. The central control room in Chicago of the Illinois Central system, controls the line all the way from there down to the Gulf of Mexico. There are just three dispatchers covering the whole of that line, with a bank of monitors that can show the state of any part of the system.

The opposite of CTC operation is Automatic Block Signalling, the equivalent to British Track Circuit Block, used on lines of two or more tracks which are each signalled in one direction only. Wrong direction moves moves require special Train Orders and Manual Block protection. ABS was used on double-track lines in the US from the early days of railways and the British principles of Absolute Block, called Manual Block, are only used in special circumstances. Therefore, unlike in the UK, there have never been intermediate block posts in the US, and every tower built has had either sidings or a junction (or, occasionally, just a flat crossing between two companies' lines) to control. Most ABS is permissive - not in the UK context of Permissive Block but more like Stop and Proceed working. This permits the passing of automatic signals when at danger, subject to certain conditions, as on the London Underground and a few other railways in the UK. Automatic Block Signalling could also be found on single lines, used in conjunction with train orders, allowing successive trains to follow at close intervals.

On the Chicago elevated system, and also in New York, signals were provided only at junctions and elsewhere drivers were required to stop their trains behind the one in front, much like the Stop on Sight principles of tram operation.

Another system of operation has superseded the use of Train Orders on single lines, called Track Warrant Control, or TWC. This is used as a more economical system than CTC on less busy lines, with the equivalent to Train Orders dictated by radio from a central control room and written out by the Engineer. This system of operation does not always have signals as the train crews will work the points (normally padlocked) when need to be looped (to cross with another train) or enter a siding. Where signals are provided, they are there only to confirm the instructions given by radio.

The timetable is critical to the operation of single lines, as trains proceed (and cross with other trains) in strict accordance with the timetable unless otherwise instructed. Many freight services are classed as Extra services (even though they run on a regular basis) because their timing can be erratic - such trains require instructions to be issued for all movement. Some staff timetables show no trains in them at all, and all traffic is treated as extras.

A similar system to TWC is Direct Traffic Control, or DTC. With this sytem, the line is divided into named blocks, or sections, usually with each block including the track from one end of a siding to the corresponding end of the next siding.  Signs showing the block names are rerected at the boundaries. The dispatcher issues authority to occupy the blocks needed to make a given move.

Train Staff operation of single lines was used at a few locations

Written Train Orders are nowadays only used in ABS areas during engineering work and emergencies when only one line is available for all traffic.

A new system, called Positive Train Separation or PTS, is under development. This would use GPS (satellite monitored Global Positioning System) to identify the position of trains in area not provided with track circuiting. As the GPS system isn't accurate enough to identify trains on adjacent lines, this is only suitable for single lines but is seen as an ideal replacement for Track Warrant Control. The system is intended to enforce safe working by applying train brakes in emergency, and avoiding what are called corn field meets - head-on collisions on single lines. PTS is also being considered as an overlay to existing signalling to control trains in the event of signals being passed at danger.


Before semaphore signals were introduced, ball signals, evolved from nautical signals, were used at key locations in New England. One example of these survives at Whitefield, New Hampshire and it has become well-known enough to gain mention on some tourism web sites. The balls indicated clear when raised, and gave rise to the now universal term of highball for a clear hand signal.

Semaphore signals were introduced in the 1870s.

On single lines, many boxes had twin-armed Train Order Signals which bore little relation to other signalling. An arm faced in each direction. This indicated to engineers (drivers) that there were Train Orders for collection. These may have been placed on a special collection pole or may be passed up manually by the operator to the engineer using a hoop on the end of a pole. This process is known as hooping up.

Power-operated signals were most commonly of the three-position, upper quadrant type. The arms were mounted to the right of the post - the opposite of British practice. Up to three arms were used for speed signalling - the top one for the Full Speed or main route, the centre one for Medium Speed or diverging route(s) and a smaller lower arm for Low Speed or siding movements.

It is thought that one of the reasons for the early adoption of upper-quadrant signals was to counteratc the effects of ice which could cause signals to fall to the clear position, a particularly dangerous situation when unmonitored automatic signals are involved.

Some US&S Type B signals used two-position home and distant arms as in UK practice, although misleadingly three spectacle galsses were provided to allow for wear.

Colour-light signals can be of the Position Light or Multiple Aspect type. Position Light signals use a number of lights of the same colour in combination to give the appropriate aspect. The principles varied between railway companies, but in its simplest form three red lamps may be displayed horizontally, inclined or vertically to give exactly the same meaning as the red-coloured arm of a three-position semaphore.


In 1998 the total surviving mechanical towers in the United States numbered a mere 40. They are an endangered species!

Additional notes by John Batts, Tim Bourne, MB, Bill Gustason, Fred Morrison, Tim Lockley and David Willoughby