Tyer's Electric Train Tablet

Tyer's Electric Train Tablet system is a form of railway signalling for single line railways used in several countries; it was first devised in Great Britain by engineer Edward Tyer after the Thorpe rail accident of 1874, which left 21 people dead.[1] It was used in New Zealand for close to 100 years until June 1994. The system used a hard disk called a tablet, a form of token.

The purpose of the system was to use the tablet as a physical guarantee to the traincrew that their train had exclusive right of way on the single line section. Without this they could not proceed beyond the section signal which protected entry to the single line.

With advances in electrical locking of the lever frame within the signal box, the tablet instrument also electrically locked the section signal lever. This was marked with a white stripe on the red background.

Construction

Main view of No. 6 instrument showing the restoring lever and the tablet tray in the "out" position

An instrument was placed at each end of the single-track section that they were to control. They were connected together electrically in such a way that operation of one would depend on operations carried out using the other.

There were various incarnations of instruments developed by Tyer & Co. Below are the more commonly found ones:

The Tyer's no. 7 tablet instrument consists of a wooden case, on top of which is a metal slide, a switch plunger, a bell plunger and an electric current indicator. On the front is an indicator which may show either of three indications: "Line Closed"; "Train Approaching" and "Train on Line". Below this there is a second slide, which has three positions: fully home, withdrawn half way, and fully withdrawn. The three indications correspond to the three positions of the slide. The case contains several tablets; they are removed from the instrument using the bottom slide, and replaced using the top slide. The instrument was so constructed that if a tablet had been withdrawn from either instrument of a pair, no further tablets could be withdrawn until the withdrawn tablet had been placed either in the other instrument, or in the same instrument from which it had been withdrawn.[2]

The Tyer's no. 6 tablet instrument is cast iron framed that has a movable drawer at the front which issues and receives the tablets. On the left hand side is a lever to reseat the tablet when it is replaced into the magazine. It has wooden side cheeks to access the complicated mechanism and a tombstone shaped wooden case on the top which houses the bell plunger, commutator and the tablet indicators for up and down trains. At the very top was the galvanometer. The signalling bell associated with the machine is separate so could be located on the block shelf or wall mounted above the instrument.

Tablets

Tablets are in the form of a disc made of metal or fibre (with a gunmetal weight at the top of the magazine), engraved with the names of the stations between which it is valid, and also provided with notches (also called configurations) or other indentations to ensure that it would fit only one pair of instruments. They were often painted in distinctive colours as an additional visual identification in areas where consecutive sections could each be controlled using the same type of instrument. The diameter of the tablets varied between the different patterns of instrument: those for the No 1 were the largest, the smallest were for the No 5 and No 7, whilst the Nos 2,3,4 and 6 all used tablets about 4.5 inches (110 mm) in diameter.

Operation

The system required each station at the end of each section to be staffed, the staff member would communicate with the staff member at the other end of the section with a bell code to release a tablet.[2]

To release a tablet at station A to send a train to station B:

To close up each machine once the train has arrived at station B from station A:

The indicators showing a tablet has been issued for an Up train from this instrument

The original Tyer's No. 1 instrument was a non-restoring design, as were the subsequent Nos 2 and 3 types. Once the tablet had been removed, it had to travel through the single line section and be replaced in its companion instrument to release the section again. This meant that should a train failure occur in the single line section or a shunt beyond station limits, the tablet had to travel to the receiving instrument by foot or horse. However, there was a way in which a shunt could be performed without removing the tablet. This was achieved by a very similar means of the staff and ticket working system.

The later No 6 model however was a restoring design, which enabled the tablet to be placed back into the issuing instrument for shunting out of station limits. This increased flexibility and did not compromise safety. It also dispensed with the written authority in the staff and ticket system. Many No 1 instruments were modified with No 6 components to make them capable of restoring also.

Areas of use

The Great Western Railway used the Tyer's No. 7 electric train tablet instrument at several locations, including the Bampton to Witney section.[3]

Sri Lanka Railways is using this system at several locations.

The Tyer's No. 6 tablet instrument was far more widespread in the UK and was adopted by most railway companies on single line installations. However, when the Tyer's No. 9 key token instrument was patented 40 years later in 1912, it began to replace the No. 6 due to it being compact, easier to use and had far fewer moving parts to service and repair.

On Britain's national network the use of Tyer's system is now almost extinct, with most lengthy single track lines having been converted to more modern systems such as Tokenless Block or Radio Electronic Token Block. However, as of 2015 the Tyer Electric Token Block (using No 7 instruments and coloured perspex tablets) remains in use on the Stranraer Line, between Girvan and Dunragit.

The last tablet

Train Advice 5685, final tablet

The last tablet used on the New Zealand Rail Network was to allow train 1602 (Wellington - Masterton morning Wairarapa Connection) to travel between Featherston and Masterton on the Wairarapa Line on Monday 4 July 1994. Even though the Masterton to Woodville tablet sections survived a few days longer, trains ran on a safe all where Train Control could allow a train to run without a tablet.

The last sections to use the tablet system were:

Heritage railways and museums

Today the tablet system lives on in many heritage railways and museums either as static / operational / interactive displays or as part of the heritage railway signalling system

Heritage railways

Railway museums

Other

Internal workings

Below are a few photographs of the internal workings. This particular instrument has had the electric line clear addition and restoring functions added at a later date. It also carries the original plates by the plunger and commutator which were often removed.

Accidents

Before tokens

After tokens

See also

References

  1. ↑ Faith 2000, p. 44.
  2. 1 2 Vaughan 1984, p. 121.
  3. ↑ Vaughan 1984, p. 102.

References

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