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Radio Electronic Token Block (RETB)

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Radio Electronic Token Block (RETB) was devised in the early 1980s as a low-cost signalling system for use mainly on lightly used single track railways in rural areas. It allows a substantial mileage of railway to be controlled from one control centre, with no physical link to the lineside infrastructure. 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. In 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 special unit inside the cab to facilitate the transfer and display of tokens. A portable version can be carried on a train that lacks 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.

RETB Installations

There have been four RETB installations in Great Britain as listed below:

Name of Line Extent of RETB Control Centre Commissioned
Kyle Line / Far North Line 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 1986
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 was replaced by the European Train Control System (ETCS) in 2010/2011, and in 2012 the East Suffolk Line RETB was replaced by Track Circuit Block. RETB is therefore now confined to lines in Scotland.

Cab Display Radio
Fig. 1: RETB Cab Display Radio.

Cab Display Unit (CDU)

On each of the original RETB installations, tokens were displayed inside the train's cab on a Cab Display Unit (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. The driver of a train that requires to enter the RETB system advises the signalman of this number, so it can be entered into the interlocking using the signalman's keyboard. The CDU number will then appear in the appropriate position on the track diagram depicted on the VDU screens. 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.

External Infrastructure

Figure 2 shows the minimum infrastructure needed at an RETB crossing loop.

Figure 2
Fig. 2: RETB Crossing Loop Layout.

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 mph applies to movements over train-operated points in both directions. Train-operated points have no facing point lock. A 'points set' indicator is provided for movements 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 crossing loop. AWS cancelling indicators applying to movements in the opposite direction from the distant boards were installed on the East Suffolk and Cambrian lines. Although cancelling indicators featured in the original Kyle Line RETB scheme, at the official inspection in November 1984 the Railway Inspectorate deemed that they had no value, leading to their removal during the following year. Thereafter, no AWS cancelling indicators were provided on RETB routes 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 were provided with a stop board, and subsequently the Scottish RETB lines were similarly equipped. 'Loop clear' markers are installed on the single line at a full train length beyond the crossing loop points.

A place where token exchanges are performed by traincrews is designated as a 'token exchange point' (TEP). A TEP need not necessarily be located at a crossing loop. An additional TEP can be established to divide the single line between two loops, so improving capacity and flexibility (see figure 3). An emergency telephone is provided at each TEP for verbal communications during failure of the radio system.

Figure 3
Fig. 3: Token Exchange Point at a station between crossing loops.

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 were locked inside cabinets located at every ground frame. The cabinets were unlocked by a key that was fastened to the CDU key. The Annett's key was attached by chain to the cabinet's interior, and this prevented the cabinet door being closed while the Annett's key was in use. Either method ensured that tokens cannot be exchanged with the ground frame released. The RETB ground frames in Scotland were altered to the Cambrian Lines method of release in 2008.

Once clear of the running line, a train can 'shut in' at a siding by returning its token after the ground frame has been restored. This allows another train to pass. A stop board is provided at the exit from a siding.

Working Procedure

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 given, 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 train has cleared the previous TEP.

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.

Types of Tokens

A section token is an ordinary token, which applies from a stop board at one TEP to a stop board, siding or buffer stop at the next TEP. It is only valid for a movement in the specified direction.

A long section token applies from a stop board at one TEP to a stop board or buffer stop at the second TEP beyond. It is equivalent to two consecutive 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 contacting the signalman. The stop board at the intermediate TEP carries a supplementary yellow sign stating that drivers in possession of a long section token may proceed. The driver is still required to inform the signalman when the train has passed the 'loop clear' marker at the intermediate TEP. Long section tokens are only available over predefined pairs of adjoining sections.

An occupied token is used to admit a train onto an occupied line at a TEP. It applies from a stop board at one TEP to the rear of a train standing at the next TEP. Occupied tokens are only available where there is an operational requirement, e.g. for the booked joining of passenger trains or for platform sharing.

An intermediate siding token applies from a stop board at a TEP to an intermediate siding situated in the section beyond, or vice versa. It is only valid for a movement in the specified direction.

A shunt token permits shunting movements or engineering work on any line between the 'loop clear' markers at opposite ends of the same TEP.

An engineering token allows the line to be occupied between the 'loop clear' markers at two adjacent TEPs. On the East Suffolk and Cambrian lines, however, they only allowed the line to be occupied between the distant boards approaching the adjacent TEPs. An engineering token can be issued to a road vehicle to enable engineering staff to carry out work on the line anywhere within the token limits. Alternatively, an engineering token can be issued to a train standing at a stop board at a TEP, prior to it entering the section beyond for engineering purposes.

On the Cambrian Lines, an engineer's full token allowed the single line to be occupied between the 'loop clear' marker at one TEP and the stop board at the next TEP. These tokens were not an original feature of that RETB installation but were a later addition.

A test token is issued to verify that the train-borne equipment is in working order. It does not give authority to enter any RETB section.

Local Operation of Remote Junctions

In RETB territory, a junction that is remote from the control centre requires a method of local operation, there being no physical control or indication links between the control centre and the lineside equipment. Working the points by mechanical ground frame is the simplest option but incurs a heavy time penalty. The alternative arrangement illustrated in figure 4 enables a more efficient method of operation.

Figure 4
Fig. 4: RETB Remote Junction Layout.

The junction site is designated as a TEP. The junction points are worked by conventional power operation and lie normally for the main line. Because these points are non-trailable, 'points set' indicators are provided for movements over them in both the facing and trailing directions. The 'points set' indicator applicable to facing movements is fitted with an alphanumeric route indicator, and a pair of driver-operated plungers is installed in a convenient position nearby. One plunger is for trains going towards the main line, and the other is for the branch line. The driver of a train about to traverse the junction in the facing direction must, on the verbal instruction of the signalman and being in possession of the correct token, press the plunger corresponding to the desired destination. This action sets the route for the train, calling the junction points as required and, providing detection of the points is made, causing the yellow light in the 'points set' indicator to illuminate along with an appropriate route indication. Once the driver has confirmed the route indication on display, the signalman may give verbal permission to proceed. If the train does not depart within a few minutes, the yellow light and route indicator will extinguish. A train approaching the junction from the branch in the trailing direction calls the points to the reverse position by the sequential operation of track circuits. The junction points self-restore for the main line route after a train has passed over them (in either direction) and cleared the relevant track circuit.

The basic arrangement shown in figure 4 does not allow for two trains simultaneously approaching the junction in the converging direction, and the corresponding tokens are interlocked to prevent this from happening. If trains arriving from the main and branch lines regularly join to form a combined service, then additional TEPs will be provided close to the junction (their stop boards usually being co-located with the distant boards for the junction TEP beyond). These TEPs need only apply in the direction of movements towards the junction. Once the first train has passed the junction points and has stopped and surrendered its token, the second train obtains an 'occupied' token at the outer TEP, authorising it to proceed and couple onto the rear of the train ahead.

Provision of TPWS

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 radio 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 disarmed. 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 TEP, TPWS loops are installed at both ends of each loop line. Most RETB crossing loops in Scotland were originally only provided with stop boards for trains departing from the loops in the usual direction. Concurrent with the installation of TPWS, new stop boards for opposite departure direction were installed as necessary so that every TPWS fitment has an associated stop board on which to mount an LSI.

RETB Next Generation

Changes to the radio spectrum at the end of 2015 forced the replacement of the remaining RETB infrastructure on the West Highland Lines and the Far North Lines. An upgrade referred to as "RETB Next Generation" has delivered new radio infrastructure and the replacement of the Cab Display Units (CDUs) with new Cab Display Radios (CDRs) (see figure 1). The new equipment supports auto-tuning, so that drivers no longer have to manually change channels as their train moves from one radio area to another.