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AN INTRODUCTION TO - page 22 / 72





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available only in Double Line sections. In single lines, a much more complicated arrangement may be required.

Absolute Block System on Single Lines

On single lines, before a train is dispatched into the block section, it must be ensured that no train is already in the section and also no train is allowed to enter the section from the opposite end of the block section. To accomplish this aim, two main types of equipments are provided on single line sections.

In the older generation of equipments, a Token was extracted from one of the Block Instruments and handed over to the driver of the train which constituted his Authority To Proceed.

Token Block Working on Single Lines:

Usually the Neal’s Ball type Co-operative Block instruments are provided. A brief introduction about its working is given in the following paragraphs.

The Block Instruments contain a number of Tokens which are spherical in shape and are also numbered serially. The mechanism of the Block Instruments and the wiring between adjacent Block Instruments is set up in such a way that at time only ONE token can be extracted from any instrument, Which means that if a token is extracted form instrument A then another token cannot be extracted from either instrument A or B. There are a total of 70 tokens provided between the two instruments and at time at least 69 tokens are always locked in the machines.

After the procedure of obtaining a Line Clear from the station in advance is obtained, a token has been extracted from the instrument; the token is handed over to the driver of the train which constitutes his Authority To Proceed. In addition obtaining the Line Clear also frees the Last Stop Signal (Advance Starter) if provided. Hence, now the driver can enter the section ahead and when he reaches the station in advance, he hands over the token to the cabinman who in turn inserts the token


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