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Radio Electronic Token Block (RETB) is a low-cost signalling system devised mainly for use on lightly used single track railways in rural areas. Very long mileages of railway can be controlled from one control centre. Fundamental to achieving this is the provision of a dedicated radio network for the transmission of verbal communications and electronic data messages between the control centre and trains. With RETB the token is an electronic message displayed inside the driving cab as opposed to the physical object used with traditional single line token systems. Every train in the RETB area must have radio equipment and a Cab Display Unit (CDU) inside the cab to facilitate the transfer and display of tokens. A portable CDU can be carried on a train that does not have permanently fitted RETB equipment. The signalman has visibility of the positions of trains on VDU screens inside the control centre. The issuing of tokens is governed by a Solid State Interlocking (SSI) which ensures that conflicting tokens can never be issued.
There are currently four RETB installations in Great Britain as listed below:
| Name of Line | Extent of RETB | Control Centre | Commissioned |
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| Far North Lines | Inverness (exclusive) to Kyle of Lochalsh, Wick and Thurso | Inverness (formerly Dingwall) | 1984-1988 |
| East Suffolk Line | Westerfield to Oulton Broad and Saxmundham to Sizewell | Saxmundham | 1985 |
| West Highland Lines | Helensburgh to Oban, Fort William (exclusive) and Mallaig | Banavie | 1987-1988 |
| Cambrian Lines | Shrewsbury (exclusive) to Machynlleth and Dovey Junction to Aberystwyth and Pwllheli | Machynlleth | 1988 |
| Table 1: Details of RETB Installations. | |||
The first line to be converted to RETB working was Dingwall to Kyle of Lochalsh, in October 1984. Originally controlled from Dingwall, control was transferred to Inverness in August 1988.
The Cambrian Lines RETB is to be replaced by the European Train Control System (ETCS) as a pilot scheme.
Tokens are displayed inside the train's cab on the CDU. The display comprises two lines of text, each up to a maximum of sixteen characters in length. The geographical limits of the movement authority are displayed when a token has been issued. The CDU has "Send" and "Receive" buttons used by the driver during token exchanges. To send or receive tokens, a CDU key must be inserted in the CDU and turned, otherwise the display will flash "Key in wrong position" when either button is pressed.
Every CDU is assigned a unique four-digit number. When a train enters the RETB system, the number is entered by the signalman into the interlocking after which it appears on the track diagram shown on the VDU screen. The CDU number identifies the train during verbal communications and is encoded into electronic data transmissions to ensure that a token can only be received by its intended train.
Figure 1 shows the minimum infrastructure necessary at an RETB crossing loop.
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| Fig. 1: RETB Crossing Loop Layout. |
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Train-operated (hydro pneumatic) points are provided at both ends of every crossing loop. These are normally set for trains to enter the loop in the normal direction of running. When a train leaves the loop, the point blades are pushed across by its wheels. After a short delay, stored hydraulic pressure restores the points to their normal position. No power supply is needed other than for points heating purposes. A standard speed restriction of 15 m.p.h. applies to movements over train-operated points in both directions. A 'points set' indicator is provided for moves in the facing direction to indicate to the driver that the points are correctly set in the normal position. The points can be manually pumped to the reverse position for shunting operations, after which they should be secured with a clamp and scotch.
A reflectorised distant board, together with an associated AWS permanent magnet, is provided on each approach to a loop. AWS cancelling indicators are installed for moves in the opposite direction, except in Scotland. At the end of each loop is a stop board worded "Stop - Obtain token and permission to proceed". On the Cambrian Lines, both ends of every loop are provided with a stop board and subsequently the Scottish schemes were similarly equipped. 'Loop clear' markers are installed on the single line, a full train's length beyond the loop points.
A location where token exchanges are performed is designated a 'token exchange point' (TEP). A TEP need not necessarily be located at a crossing loop. An additional TEP can be established to split a long section between two loops so that capacity is improved (see figure 2). An emergency telephone is provided at each TEP for verbal communications during failure of the radio system.
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| Fig. 2: Token Exchange Point at a station between crossing loops. |
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A connection into a siding is worked from a ground frame released by Annett's key. In Scotland, the Annett's keys were originally attached to the CDU keys, one of which is carried on every train. The Annett's key could unlock any ground frame on the RETB system. On the Cambrian Lines, individual Annett's keys are locked inside cabinets located at every ground frame. The cabinets are unlocked by a key which is fastened to the CDU key. The Annett's key is attached by chain to the cabinet's interior and this prevents the cabinet door being closed while the Annett's key is in use. Either method ensures that tokens cannot be exchanged with the ground frame released. In 2008, the RETB ground frames in Scotland are being altered to the Cambrian Lines method of release.
A train can 'shut in' at a siding by returning its token, thereby allowing another train to pass. A stop board is provided at the exit from a siding.
Before passing a stop board, the driver must be in possession of the correct token and have had verbal authority from the signalman to proceed. The driver will speak to the signalman to request the token and press the "Receive" button on the CDU. Following successful transmission of data, the token will be displayed on the CDU. The driver will advise the signalman that the token is on display and request permission to proceed. Once verbal permission has been granted, the train has authority to travel to the far end of the section.
On departing the TEP, the driver must advise the signalman when the whole train has passed beyond the 'loop clear' marker. The signalman enters this information via the keyboard to inform the interlocking that the loop is clear for a following train to approach.
On arriving at the stop board at the end of the section, the driver will contact the signalman and press the "Send" button on the CDU to return the token. The driver will advise the signalman when the CDU display goes blank. Although no token is displayed on the CDU, the interlocking remembers the position of the train and will not allow the issue of any token that would let another train onto the occupied track.
Token exchanges must only be made while the train is at a stand.
A section token is an ordinary token, which applies from the stop board at one TEP to the stop board at the next TEP. It is only valid for a movement in the direction stated.
A long section token applies from the stop board at one TEP to the stop board at the second TEP ahead. It is equivalent to two ordinary section tokens. When a long section token is issued, the driver is permitted to pass the stop board at the intermediate TEP without stopping or having to contact the signalman. These stop boards carry a supplementary yellow sign stating that drivers in possession of a long section token may proceed. The 'loop clear' procedure must still be carried out on passing the 'loop clear' marker at the intermediate TEP. Long section tokens are only available over defined pairs of adjoining sections.
An occupied token is used for joining passenger trains. It applies from the stop board at one TEP to the rear of the train standing at the next TEP.
An intermediate siding token applies from the stop board at a TEP to an intermediate siding situated in the section ahead.
A shunt token applies over all lines between the 'loop clear' markers at a TEP.
An engineering token applies between the 'loop clear' markers of two adjacent TEPs. On the Cambrian and East Suffolk lines however, it applies between the distant boards (see below). It is used during maintenance activities.
On the Cambrian Lines, an engineers full token applies between the 'loop clear' markers of two adjacent TEPs. These tokens were not an original feature of the installation and were added subsequently.
A test token is issued to check that the equipment is in working order. It does not give authority to proceed.
The most significant development on RETB lines since commissioning has been the fitment of the Train Protection & Warning System (TPWS) in 2003. This has greatly increased the amount of external infrastructure. Each TEP where TPWS is fitted requires an uninterruptible power supply, a 'Trackside Radio Control Unit' (TRCU) and a mast supporting two RETB antennae. The equipment will 'listen in' to the exchange of tokens between the control centre and trains over the radio and determine which TPWS loops at that TEP should be energised and which can be de-energised for an authorised movement. A 'Lineside Status Indicator' (LSI) is mounted on each TPWS-fitted stop board. This comprises a blue light which is normally steady but which flashes when the associated TPWS is de-energised. A repeater LSI may be fitted to the distant board in rear if the stop board can be approached at speed, e.g. at an intermediate TEP without a crossing loop.
At a crossing loop, TPWS loops are installed at both ends of each loop. In Scotland, stop boards did not originally exist at both ends of every crossing loop. Extra stop boards have had to be installed to avoid a situation where TPWS loops were fitted without there being an associated stop board on which to mount an LSI.
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