Modernisation of the Cambrian Lines 1988


Modernisation of the Cambrian Lines 1988

by Alan Jones


Stop board

Radio Electronic Token Block (R.E.T.B) signalling offers opportunities in improved train control and communication for the rural network.

The Cambrian Lines of Wales from Shrewsbury (Sutton Bridge Junction) to Aberystwyth & Pwllheli are controlled from a control centre at Machynlleth Signalbox. The scheme was brought into use on October 24th 1988.

Fundamental to the concept of R.E.T.B signalling is the need to have extensive radio coverage from the control centre to the trains working within the area of control.

Each locomotive and sprinter unit in use on the line is equipped with a radio to provide communication via the dedicated radio bearer network to the control centre. In addition the driver has a portable Cab Display Unit (C.D.U) which interrogates and displays electronic token data messages sent from the control centre to the locomotive. Each C.D.U. has a unique serial number, which is used as the identification address for messages.

The signaller at the control centre has a Visual Display Unit (V.D.U) schematic diagram of the lines under his control. This V.D.U indicates the position of trains (using the C.D.U. identification number) and the extent of any “electronic token” authority used. The V.D.U display is driven be a Solid State Interlocking (S.S.I) which holds all the safety data.

A keyboard allows the signaller to input the relevant data instructions into the system, and a small radio console enables him to communicate verbally with the drivers and other users of the system.

At lineside, the signalling equipment is much simplified by the use of reflectorised marker boards and by the use of train operated hydropneumatic point mechanism.

A concentrator is provided for communication facilities to stations, depots, offices and level crossings etc, as well as to emergency telephones at passing loops.

RETB Signalling Operation

Stop board with
long section token board

Before a train enters service, and after connecting up the C.D.U, the driver requests a TEST TOKEN from the signaller. After satisfactory receipt, the TEST TOKEN is returned. If the test is not satisfactory it is attempted from the rear cab.

Each C.D.U. has a unique four-figure number. When the train requires to enter the area controlled by electronic token working, the driver calls the signaller and gives his location, reporting number and C.D.U. number. The signaller enters the C.D.U. number into the interlocking with the geographical location of the train.

Once entered into the system the driver requests a token to the next token exchange point.

Entering a radio number into the system and issuing or returning a token requires the co-operation of the signaller and the driver. The driver uses his radio to initiate a verbal request for the “activity”.

The signaller confirms the verbal request and generates the data message by keying the “activity” into the console. Data transmission commences when the signaller presses his enter button. Provided the “activity” entered is valid and provided the driver holds down the appropriate send or receive button, the token data exchange will take place.

After receiving an electronic token (which is displayed on the C.D.U.) the driver gives a verbal acknowledgement of correct receipt of the token and requests verbal confirmation of authority from the signaller to proceed.

Once the train passes the loop clear/station limit board and enters the single line section, the driver reports this verbally to the signaller who enters the fact on his console so that the interlocking and V.D.U diagram is updated.

A single headed arrow on the diagram reminds the signaller of the authorised direction of passage of the train, which is represented by its C.D.U. number in a box.

The signaller uses an emergency key where non-standard operation is required, use of the emergency procedure is authorised by the controller. The signaller and controller record use of the emergency key as well as the update given on the V.D.U and printer.

In addition to the V.D.U diagram, the signaller has a hard diagram of the whole line on which he can place magnetic markers to represent the locations of trains and the sections occupied. He would use this magnetic diagram in the event of a failure of the V.D.U equipment.

Station limits board

In the event of complete radio failure, emergency telephones are provided at each of the token exchange points. By using the phone, the driver can request special authority to proceed. The driver under the signaller’s direction completes special authority cards. Before the driver is authorised to proceed the signaller has to secure the route by entering TRACK BLOCKED on his V.D.U or by laying TRACK BLOCKED magnetic markers on his diagram.

The basic radio / token exchange procedure requires three messages, viz. TOKEN ISSUE, LOOP CLEARANCE and TOKEN RETURN. If the line ahead is clear for two sections the facility exists for the signaller to issue the driver a LONG SECTION TOKEN. The limits of these long section tokens are fixed so that driver’s do not get confused as to the limits of the section. Long section tokens are not available through passing loops.

The following types of token are available for the signaller to issue: –

  • Section Token – Authority to proceed from the Stop Board at one Token Exchange Point (T.E.P) to the stop board at the next T.E.P.
  • Long Section Token – Authority to proceed from the Stop Board at one (T.E.P) to the stop board at the next but one T.E.P.
  • Shunt Token – Authority to shunt between the Station Limits Boards at the same T.E.P.
  • Engineers Token – Authority for an Engineer to occupy the section of line between the Distant Marker Boards for engineering purposes.
  • Engineers Full Token – Authority for an Engineer to occupy the section of line between the Station Limits Board applicable to one T.E.P. to the Stop Board at the next T.E.P. for engineering purposes.

Machynlleth Control Centre

Station limits board

The route is divided into three separate signalling sections, each with its own independent equipment and area of control. The East section controls the line from Sutton Bridge Junction (exclusive) to Machynlleth (exclusive).

The West section controls the line from Dovey Junction (exclusive) to Aberystwyth (inclusive) and the branch from Dovey Junction to Pwllheli (inclusive).

The section of line from Machynlleth to Dovey Junction is controlled by the East signaller via the existing mechanical lever frame, the method of signalling for this section is T.C.B single lines. Thus by installing the two control desks on the operating floor adjacent to the lever frame, the entire route of approximate 135 miles is now centrally controlled from Machynlleth Signalbox by two men.

The Radio Network

Channel change board

The radio network allows communication between the signaller and any mobile user of the system, whether it is a train or other staff. Communication is “open channel” in that all users on either the East or West system can listen in to all messages being transmitted on that system. This means that users can also talk to one another, independent of the signaller where regulations permit.

A series of radio base stations are located through the area to be covered. Although radio coverage may not be continuous throughout the area, attention is given to ensuring good radio coverage at normal user locations i.e. where token messages have to be exchanged with the signaller. In many instances these base stations share sites with other users, e.g. B.B.C e.t.c. These base stations are usually located on a hill top site to give as wide an area of coverage as possible.

Each base station is allocated a specific frequency / channel number. The Cambrian scheme operates on the Band III group of frequencies. For user purposes the channel identification is a three-digit number, which are in the range from channel 101 to channel 125.

The user must select the appropriate channel for the geographical area of coverage that he is in. For train drivers the changeover points are marked with Channel Change boards at the lineside.

The base station aerials are usually folded dipoles, to give a broad spread of coverage. Some directivity is required at certain sites to avoid interference from other users of Band III.

Points indicator light

To provide the link between adjacent base stations and back to the signaller a repeater station is provided. This repeater works in directional mode (i.e. it cannot transmit in both directions at the one time). At the same time the repeater provides the frequency changing to match the radio channel frequencies of the adjacent base station.

In most instances the repeater sites are located at the lineside. Wide coverage is not required from the repeater sites, as they only receive and transmit to specific base stations. This means that directional aerials and lowered powered transmitters can be used.

The signallers have direct access to their respective radio networks from the radio units at Machynlleth Signalbox.

At the end of the systems a standby dial-up radio unit is located. In the event of a partial failure of the network the signaller can dial up this remote end and access the network from the dial-up unit. An alarm indication is given to Machynlleth via the radio network if the power supply is lost at any station.

The Mobile Equipment

Differential speed board

On the Cambrian two forms of mobile equipment are in use – one for service trains and one for road and rail vehicles in use by the Civil Engineers in that area.

Because the service rolling stock is not captive to the Cambrian, the system differs to that used on the West Highland R.E.T.B whereby on the Cambrian portable C.D.U’s are used.

The Lineside Equipment

In modern power signalling systems the benefits of centralised control are achieved by remote operation of the signals and points from the central signalling centre. This solution is often too expensive for rural lines. Under R.E.T.B operation, the lineside signalling can be simplified because the electronic token in the cab and the radio communication to the signaller give a form of cab signalling.

Reflectorised boards replace the lineside signals.

A STOP board indicates the end of each token section and reminds the driver to obtain a token for the next section before proceeding.

A DISTANT board gives a warning of the approach to a loop (or T.E.P.) An A.W.S magnet is provided on the approach to the distant board to help drivers identify his position in conditions of poor visibility.

Advance warning board
(St George’s Cross)

A LOOP CLEAR / STATION LIMITS board is provided approximately one train length clear of the loop points (or T.E.P.), This board reminds the driver to contact the signaller and confirm that he has cleared the previous token section. It is also used to indicate the limits of authority for shunting movements “within station limits” or the limits of possessions for engineering work.

All points at passing loops are made self-restoring. A hydropneumatic mechanism is used to store the pressure needed to hold the point switchblades in the correct position for facing movements. When trains pass over these points in the trailing position the hydropneumatic forces increase and assist the self-restoration of the points after the train has passed clear. A separate locked manual hydraulic pump unit can be used by train drivers (or other staff) to pump the points over to the reverse position.

A POINTS INDICATOR LIGHT (yellow light) is provided on the facing direction approach to the points. When lit, this gives an indication to the train driver that the points are closed, the circuit being taken through electric detection at the points.

At Dovey Junction the junction points are powered operated, with selection of the route (Aberystwyth or Pwllheli) being made by the East signaller by operation of the appropriate levers in the frame at Machynlleth.

Track circuits and signals are used to continuously monitor and control all movements between Machynlleth & Dovey Junction.

Where siding connections are provided at passing loops, these are ground frame operated. An Annett key lock secures all the ground frames. Each driver carries a cupboard key, which is secured to the C.D.U. installation key. This helps to remind the driver to obtain the correct authority to use the siding. Removal of the C.D.U. key (when operation of the ground frame is required) ensures that no further token authorities are issued to that C.D.U. until the keys are returned.

The cupboard key enables him to gain access to a steel cabinet adjacent to the frame. This cabinet houses the Annett’s key, secured by a stout metal chain.

This chain prevents the cupboard door from closing when the key is in the frame.

A simple but effective mechanical interlocking prevents the cupboard key (and hence the C.D.U. key) from being withdrawn until the Annett’s key is replaced.

Solid State Interlocking (SSI)

AWS cancelling board

The S.S.I equipment used in R.E.T.B schemes is similar to the electronic interlocking equipment used in S.S.I colour light signalling schemes. For R.E.T.B operation the S.S.I Main Processor Modules (M.P.M) are memories programmed for R.E.T.B principles of working, together with the relevant geographical data for the specific scheme.

The M.P.M units are provided for each system and these work in a “two out of three” mode. If one of the three M.P.M’s fail, or its output disagrees with the other two and it shuts down, the system will remain running with the other two M.P.M’s until the third is replaced.

The plug connections for each interlocking processor are hard wire coded to ensure that only the correct M.P.M for that rack can be used.

Messages from the signallers keyboard are interpreted by the Panel Processor Module (P.P.M) which interfaces with the M.P.M’s. The P.P.M is also used to drive the V.D.U display and provide the output information to the signaller.

In colour light signalling schemes the S.S.I output to the lineside is via Data Link Modules (D.L.M) which feed out to the various signal and point modules. On the Cambrian scheme, a D.L.M and a Signal Module is provided on each system to provide the interface with the conventional control and indication relay circuits for the section of line Machynlleth to Dovey Junction, as well as the release for Sutton Bridge Home signal (the latter being via a radio link)

A communications processor is used to free the output from the M.P.M’s to the radio equipment control rack.

Distant marker board

Before transmitting a token the S.S.I carries out several logic checks, including:-

  1. That the C.D.U. number is entered into the system and that it is unique.
  2. That the token is being issued to a berth already occupied by that number.
  3. That berths required free are not occupied by another C.D.U. number.
  4. That the train’s berth is where the interlocking expects it to be.
  5. That the signallers console key stroke is correct and that no spurious data is present.

The printer in the signalbox records the passing of tokens to and from the trains. This is driven by a panel processor.


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