|The operation of a service on a single line of track is
fraught with potential danger, so there are plenty of rules, regulations and
solutions to make the single line a safe place to be. As with our other
Signalling articles, this is not meant to be an in-depth tutorial but more a
general introduction to the whys and wherefores.
It goes without saying that the object of this particular exercise is to prevent any possibility of a collision between trains following each other, or travelling in opposite directions, on a single line. At its most basic there is the 'One Engine in Steam' system. Only one engine (or MU, or engines coupled together) allowed on a section at any one time. In the very early days this was regulated by the use of a Pilotman who travelled on the engine but this was soon replaced by using a train Staff that the driver had to have in his possession at all times whilst in the section (unless some emergency occurred requiring assistance to be rendered to the train). This method had its drawbacks as trains had to run alternately, first in one direction, then in the other, as otherwise the Staff would be at the wrong end of the section. This basic method of operation was best suited to locations where the service always runs in alternate directions, in the main on dead end branches. The Southern Railway operated several lines using OES, for example between:
The driver would have in his possession the Staff, in the early days made of wood, later on of metal, on which would be engraved the names of either end of the section.
To get round the problem of the likelihood of the train Staff being at the wrong end of a section the system was developed first into the Staff and Ticket system, then into the Electric Tablet, Staff or Token system. The Staff and Ticket system was made possible with the introduction of the electric telegraph and block working. Now that quick and convenient communication could be made between the two ends of a section it became possible to send two or more consecutive trains through a section. The Staff for the section would be obtained but instead of being given to the first driver he would be shown the Staff and presented with a written authority (the 'Ticket') to proceed. Once confirmation was received from the other end that the first train had safely left the section the second driver (if three or more trains required to run) would be shown the Staff and and presented with a Ticket. This could, though highly unlikely, continue for many trains until the last train, the driver of which would carry the Staff through the section. This system, although a great improvement, was not foolproof as a misunderstanding could lead to a following train being allowed into the section before the preceding one had left it.
The next big step forward came in 1880 when the Caledonian Railway and Tyer & Co developed a system whereby a number of Tablets (in the form of discs) were held in two instruments, one at each end of a section, which were electrically interlocked so that only one might be withdrawn at any one time. This great leap forward meant that the Staff and Ticket system was no longer required as a Tablet could be withdrawn any time the section was not occupied, and for travel in whichever direction was required. To avoid confusion between adjacent sections, Tablets were made with holes of different shapes so that the Tablet for one section could not be put into the instrument for another and so that the driver could more easily identify the Tablet being passed to him. Various other systems followed, but all working to the same basic principle, which involved other forms of Tablet, Token or the Electric Train Staff, a metal staff in place of the disc of the Tablet or Token. Tokens and Staffs and were painted in different colours as well as having different configurations to prevent confusion betweem adjacent sections, though knowing the correct colour did not remove the obligation for him to check that the correct ends of section were engraved on the Staff or Tokent
|Tyer's Nº6 Tablet used on the Meon
Different configurations are achieved by varying the shape of the hole and notch.
|Electric Train Staff as used to be used on the
Different configurations are achieved by a variable distance between 4th & 5th rings and shown by the colour of the handle.
|Electric Key Token as used on the Bluebell
Different configurations are achieved by varying the 'key' end, and shown by the shape of the hole and colour of the handle.
photograph by Chris Osment
photographs by Peter Richards
|These systems remained in use over many years and are
still in use today on our preserved railways, though just token systems and a
couple of tablet systems remain in use on Network Rail. The problems of
exchanging Staffs in particular and Tokens (carried in a variety of designs of
leather pouch, affixed to a hoop) between Drivers and Signalmen led to other
inventions. First there were various forms of automatic Token exchange which
meant that trains did not need to slow to 10-15 m.p.h. for the exchange and
then, in the 1960s, the tokenless block was introduced. Developed initially by
British Railways in Scotland and subsequently on the Western Region, this soon
spread to the Southern Region where a system was developed for use on the Isle
of Wight. More recent tokenless systems rely on the use of full track
circuiting, though with either version the Driver's authority to enter any
particular section is given by the lowering of the section starting signal.
Recent developments have involved the use of radio. Radio Electronic Token Block was developed primarily as a cost reduction exercise as it does away with the need for interlocking and frequent Signalboxes. The Token takes the form of an in-cab display which provides the authority to proceed, issued from a central computer.
This page was last updated 4 January 2004
Example of Electric Train Staff Instrument | Examples of Electric Key Token Instruments | Examples of Electric Train Tablet Instruments
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