BR Automatic Train Control System (AWS)

Standard

In 1948, the Railway Executive made a recommendation to the British Transport Commission to adopt, in principle, the application of a system of automatic train control. The term “Automatic Train Control,” although that was its official title, is probably somewhat misleading, since it does convey the idea of total automation and that, as we know today, is something entirely different, The ATC system subsequently adopted on British Railways provided for a visual and audible warning of the position of a distant signal, and where the latter was at caution, if the indication in the locomotive cab was not cancelled by the driver, an automatic brake application was made.

There were prior to 1948, two other ATC/AWS systems already in use, that on the GWR, introduced there in 1912, and a system, similar in many respects to the later BR standard type, installed by the LMSR on the LTS line in 1938. In keeping with the Railway Executive’s principle of standardising combinations of the best practices from other regions, experiments of various kinds were carried out between 1948 and 1952 on potential ATC/ AWS systems. On the face of it, only having two practical systems with which to effect comparisons, the task would seem to have been relatively straightforward. However it appears that a number of technical difficulties arose during this time, and then, if one is to believe the report of the inquiry into the disastrous Harrow & Wealdstone accident of October 1952, there was the question of snow and ice in the north. 1n point of fact it was the cause of this accident which both laid emphasis on the need for, and gave additional impetus to, the development of British Railways Standard ATC system.

BR ATC DiagramBR ATC Maintenance InstructionsThe prototype apparatus was installed during 1952 over the 43 miles between New Barnet and Huntingdon on the Eastern Region main line. The first locomotive to be equipped with the new ATC apparatus was the new Peppercorn Al Pacific no. 60150 Kestrel, which on 17 October 1952 took the 3.10 p.m. from Kings Cross on the first of a series of tests. The testing took some considerable time, for it was not until 1957 that the authorities were entirely happy with the system, and then declared it to be adopted for standard use on all regions of British Railways, the GWR system notwithwithstanding, although much of the latter was replaced by the BR standard system in later years. It has now been fitted to almost all traction units and extended to cover most of the important lines throughout the BR network; the title being altered to Automatic Warning System in order to give a more precise indication of the system’s function.

Operation of the system was based on the use of magnets situated in the permanent way between the running rails, with the tops of the magnets at rail level. The designed purpose of the system was to give the driver audible and visual indication of the position of a distant signal, 200 yards in advance of the signal. The two magnets were placed 2ft 6ins. apart, centre to centre, the one furthest away from the signal being permanent, the nearer being electrically activated when the distant signal was held in the clear position, and dead when at caution. If the indicators were overrun with the signal at caution and the driver had not cancelled the indication, an application of the brake would be made since the apparatus was connected to the automatic brake. There were a total of four indications of the position of a distant signal that could be given to the driver, two visual and two audible. The audible indications were given by a bell or a horn; the former ringing for two seconds on passing over the inductor of a distant signal at clear, and the horn was actuated after a delay of one second on passing over the inductor of a distant signal at caution. The two visual indications were provided by the Driver’s Control Unit (DCU). On passing over the permanent magnet of a signal at caution, the visual display would be changed from yellow and black to all black; then on passing over the electromagnet the horn would be sounded and an automatic brake application made after a delay of three seconds. Re-setting the equipment by the driver would return the display to a yellow and black aspect. If the signal being approached was off, or at clear, the same procedure would take over the perm- anent magnet although, since the electromagnet would in this case be energised, on passing it the DCU display would remain black and a bell would ring for two seconds. In point of fact the DCU would display a black indication until a distant signal set at caution was approached, following which, operation of the re- setting device would change the display to yellow and black.

BR ATC fitted 78036 - Photo 692 copy

BR Standard Class 2MT 2-6-0 in this view, shows the battery box on the running boards, immediately in front of the cab spectacle plate. Photo courtesy: Lens of Sutton

Locomotive Equipment.

The apparatus provided on the track was basically very simple, consisting essentially of the two inductors. Locomotive equipment however amounted to a total of some ten separate items and associated cabling and pipework. Since it was intended for use on steam locomotives, the apparatus had to be specially designed to withstand the extreme conditions to be met with: smoke, steam, heat, vibration, shock etc. The equipment was further designed about two basic parameters:-

  1. From one shopping to the next, all items should function without maintenance from the shed staff.
  2. Should one item fail, it could be replaced without requiring any disturbance to the electrical wiring.

The sensing device, or receiver, was mounted on a stretcher immediately in front of the leading coupled wheels and positioned centrally between the frames at a height above rail level of about 4 Y, and 6 Y, inches. The height of the receiver was not over critical, hence the allowable range of movement of two inches although once fixed no provision was made for adjustment. The receiver was basically a polarised relay, actuated by the track mounted inductors, to transmit electric current to the Relay & Cab Junction Box. This was effected through its own junction box mounted on the frame, to which it was attached through a flexible connection. Flexible connections were provided between the receiver junction box, receiver and relay unit, to allow for any relative movement between the engine frame and receiver. The relay and cab junction box, or relay unit, could be described as the nerve centre of the apparatus, to which all cables from the other items of equipment were connected, its function being to translate the electrical information into audible and visual indications of the signals and where appropriate to initiate the application of the brake. Perhaps the second most important item was the Driver’s Control Unit (DCU) on this unit, the signals being displayed to the driver visually. The unit, mounted in the cab on the driver’s side contained an electro-pneumatically operated solenoid valve, indicator and resetting handle. The solenoid valve, being connected to both electrical and pneumatic circuits and when not activated, under normal conditions ensured that the feed to the horn was maintained at normal air pressure with the brakes off, via the timing and ATC reservoirs. The last key item of equipment on the engine was the ATC Brake Valve, through which the actual automatic application of brake was made. The Brake Valve consisted of a diaphragm acting on a flat disc whose centre was attached to a spindle operating the main brake valve. The valve was normally closed to atmosphere, one side being attached to the train pipe and the other to the Timing Reservoir. Application of the brake was made by admitting air at a controlled rate through the Timing Reservoir, lifting the diaphragm and opening the valve to admit air to the train pipe. A plug on the valve, which would normally be sealed in the open position, could be screwed down to close it in the event of a failure. The remaining items of equipment could probably be classed as ancillary, except perhaps for the ATC Vacuum Reservoir, whose function was to maintain vacuum in the Timing Reservoir and to equalise pressure between train pipe and ATC side of the brake valve.

BR ATC fitted 70033 - Photo 917 copy

BR Standard Class 7MT Britannia Pacific No. 7033 “Charles Dickens” in original guise, with handrails around the smoke deflectors, also shows the battery box on the running boards, immediately in front of the cab spectacle plate. Photo courtesy: Lens of Sutton

On the whole, it is not necessary to describe all the individual items of the Standard AWS apparatus at this stage, since it is the broader principles of the operation of such devices that concerns the majority of us. In retrospect it is interesting to note that much of the equipment, including the specialised electrical devices, was perhaps crude and bulky when compared with equipment of today.

As previously indicated, it was not until 1957 that British Railways decided to adopt as standard the form of AWS described here and eventually extended to cover most important lines on the system. It was intended to fit all traction units with the equipment. Though this has largely been accomplished, quite a number still remain in service unfitted. The GWR’s original electro- mechanical method of AWS has also now succumbed to the standard arrangement, removing the last traces of individuality of that region. Doubtless though it will not be long before the Standard AWS is superseded by a more sophisticated arrangement further to improve signalling and train control.

References.


  1. BR London Midland Region Magazine – November 1952.
  2. BR Automatic Train Control: Maintenance Instructions [BR 31168/2).
  3. BTC Handbook For Steam Locomotive Enginemen.
  4. See also: Kempe’s Engineers’ Yearbook Vol. 1.

 

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