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. 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.

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 Below this there is a second slide, which has three positions: fully home, withdrawn half way, and fully withdrawn.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 The instrument was so constructed that if a tablet had been withdrawn from either instrument of a pair, no further tablets could be withdrawn until the withdrawn tablet had been placed either in the other instrument, or in the same instrument from which it had been withdrawn.

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.

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Wikipedia