HomeSignalling and TelecomsSingle Line Internet Control

Single Line Internet Control

Listen to this article

The safe operation and signalling of single-line railways are crucial as any head-on crash between two trains is potentially catastrophic. Incidents of this nature are thankfully very rare as the systems devised to allow only one train into a single-line section are robust and well respected.

Train drivers have to be assured, beyond any doubt, that it is safe to proceed into the section ahead. Initially this was achieved by possession of a single physical ‘staff’ for the section, later developed into tablet or token machines to give more flexibility. In more modern times, No Signalman Key Token (NSKT) operation has emerged, whereby the drivers actually operate the token machines, mainly at passing loops, instead of a signaller.

Tokenless Block is another alternative, where track circuits give signallers full viewing of the single line section to clear the signals accordingly. Use of radio in the RETB (Radio Electronic Token Block) application has also been developed.

Tablet or token machines require a landline connection between adjacent signal boxes, never a problem in the past but it can be a constraint nowadays, with the continuing demand to rationalise lineside infrastructure. So, can other means be used to connect the token instruments?

Park Signalling, now a member of the Unipart Rail group, has developed a system whereby secure IP (Internet Protocol) communication is used for the connection, either over the internet or any other non-dedicated communications link such as 4G cellular radio or Network Rail Telecom’s FTNx network. This represents an innovative means of adapting 21st century technology to link with Victorian engineering.

Rail Engineer went to the Ecclesbourne Valley railway at Duffield to view a demonstration of the system.

Tyer token machine

The Tyer token machine

To understand the operation, it is necessary to know how a traditional token machine works. It has five basic components – a plunger (1) for transmitting bell signals to adjacent instruments, a pointer (2) for indicating to the signallers the state of the token section, an indicator (3) to show the outgoing and incoming signals sent via the plunger on the machines at each end of the section, and a commutator (4) in which the tokens are inserted when taken out or put back into the magazine (5) for holding the tokens.

Operation consists of the signaller at one end sending a bell code to the signaller at the far end. If the section is clear, the far end signaller keeps the plunger depressed whilst the near end signaller slides a token up from the magazine and into the commutator, turning the token anti-clockwise to remove it from the machine. The commutator turn reverses the line polarity to the far end machine which electrically prevents a token being removed from that machine or from removing a further token from the originating machine.

The token is put into a leather pouch and handed to the driver who checks that it is correct for the section ahead. Once the train has reached the far end, the driver hands the token to the signaller who puts it into the commutator turning it clockwise, which then allows the instruments to be back ‘in phase’ in readiness for the next token extraction.

In short, only one token can be removed at any one time for the particular single line section.

DiBloC – Digital Block Controller

This is the fancy name for the almost lookalike new token instrument. In designing the machine, Park Signalling took a fresh look at some of the features in the old unit to decide whether they were still required. As such, the operation is similar but not identical to what has gone before.

DiBloC – Digital Block Controller

The shape and size of the machine is the same, including the magazine for holding the tokens, but is now made in aluminium. The front panel is slightly different in both facilities and operation. The bell plunger and the pointer for indicating the token state are replaced by an illuminated push button for ‘Request Token’, a ‘Signallers Release’ button/light for when permission is given for a token to be extracted, plus two LED lights to indicate ‘Token Available’ and ‘Token Not Available’.

The commutator is similar but its position is read optically to interface with the electronics that lock or unlock the insertion or removal of a token. Insertion of a token is not dependent on power being available. The token key ‘cut’ is compatible with any existing tokens and any replacement project can retain the original tokens, thus providing the traditional safeguard against trying to insert the ‘wrong’ token into the machine.

Completing the front panel are a series of LEDs to indicate power failure, network fault, building intrusion and suchlike.

Behind the front plate is a printed circuit board with duplicated processors and electronics that control the instrument, including the mechanical aspects and electrical shotbolts. This is linked via a multiway cable to an internet router which, at Ecclesbourne, was located in an equipment cabinet underneath the token instrument but could be at any convenient office location. This router is not unique to the machine and may provide the internet connection to other computer equipment at the location.

The IP (Internet Protocol) address is assigned a static identity and provides a single network channel. For resilience purposes, two fully independent network channels should be made available to guard against one channel failing. The power is 24V DC and can incorporate an uninterruptable power supply (UPS) back-up battery if required. The weight of the unit is around 27kg.

The connection from the router to the outside world can be either by fibre, 4G cellular network, satellite or to a rail company’s own telecom network. The system is designed to allow up to six token machines in a group. This will cater for situations where, for instance, a terminus location may have a machine on two or more platform ends or where an intermediate machine may be required to lock a train into a siding or freight yard and the token given up.

The IP arrangements will be similar to creating a group address list with messages being sent in a loop from machine 1 to the next machine and so on until the last one sends it back to machine 1.

DiBloC in operation

The system can work in various modes:

  • Signalbox to signalbox, where two signallers extract and return the tokens;
  • Signalling centre to other locations (typically passing loops) where the driver will activate the token machine;
  • ‘No Signaller’ operation with drivers operating the machines, located normally at passing loops, akin to NSKT operation.

To extract a token, firstly the ‘Token Available’ green LED must be lit and the ‘Request Token’ button is pushed. This sends a communication to all the other machines in the group and, if no token is already out, the machine indicates that a token can be released. The token is then lifted from the magazine and rotated out of the commutator. All the machines in the group will then be informed that a token has been extracted and the red LED ‘Token not Available’ is lit.

Once the driver has checked that the token is correct for the section ahead, the train can proceed.

At the far end, the token is inserted into the commutator and turned, with the optical reader recognising that a token replacement is occurring, thus sending an internet message to the other machines that a normal state has resumed and the ‘Token Available’ LED is again lit.

Where a form of centralised signalling control is in operation, the driver who requires a token presses the ‘Request Token’ as before, but this sends an internet message to the signal centre. If the signaller is in agreement to a token being released, they will press the ‘Signallers Release’ yellow button, which will initiate an internet message to the requesting token machine where an audible alert will sound, indicating that the request has been granted and the driver can now extract the token. Such a situation will exist where train regulation and timetable adherence is such that a train may be held at a location whilst another train is allowed into the section first.

In the UK, the operating rules require that verbal permission from the signaller is obtained before any token extraction takes place so that both parties understand the requirement of that particular train movement. In other countries, where the overall route management is less formalised, it is possible that a ‘first come, first served’ token extraction will be allowed, although this can cause timetabling problems if trains are running late.

Commercial considerations

The concept was initiated by a Park Signalling engineer who also worked as a volunteer on a heritage railway and recognised the problems of providing traditional telecom line connections. Many heritage lines use traditional token instruments and Network Rail still has around 110 token machines in service. There are also many countries in the world that have been sold Tyer token machines down the years and where maintaining reliable lineside cabling is a challenge.

The market is not huge but is considered worthwhile to make the development investment, which was supported by the Birmingham Centre for Railway Research and Education.

The first application will be on the Ecclesbourne Valley line, where the staff can test and evaluate the system and tease out any weaknesses. A second application may well be at the southern end of the Central Wales line (Shrewsbury to Llanelli) where NSKT operation is the mode of operation.

Reaction to date is encouraging but with some potential users asking if the bell push plunger and the ‘train going or train coming’ indicator can be incorporated. The answer is yes, but it will have cost implications and the modification would likely be bolt-on units. Users are asked to consider whether, in these days of cellular telephony, such add-ons are really necessary.

Clearly, the machines are precision made and the design has incurred development costs, so investment is required when replacing existing units. Park Signalling is mindful of this and will offer various hire purchase or rental options, as well as an installation service which can include maintenance and fault finding. An arrangement with SigNet Solutions at Derby can give training to prospective users.

The system incorporates SIL 3 safety functions and is designed to EN 50126, 50128 and 50129 standards with certification expected shortly. Network Rail guidelines for cyber security are followed.

Thanks to Ian Allison, Robin Lee and the engineers at Park Signalling for a most interesting day.

Clive Kessell
Clive Kessell
SPECIALIST AREAS Signalling and telecommunications, traffic management, digital railway Clive Kessell joined British Rail as an Engineering Student in 1961 and graduated via a thin sandwich course in Electrical Engineering from City University, London. He has been involved in railway telecommunications and signalling for his whole working life. He made telecommunications his primary expertise and became responsible for the roll out of Cab Secure Radio and the National Radio Network during the 1970s. He became Telecommunications Engineer for the Southern Region in 1979 and for all of BR in 1984. Appointed Director, Engineering of BR Telecommunications in 1990, Clive moved to Racal in 1995 with privatisation and became Director, Engineering Services for Racal Fieldforce in 1999. He left mainstream employment in 2001 but still offers consultancy services to the rail industry through Centuria Comrail Ltd. Clive has also been heavily involved with various railway industry bodies. He was President of the Institution of Railway Signal Engineers (IRSE) in 1999/2000 and Chairman of the Railway Engineers Forum (REF) from 2003 to 2007. He continues as a member of the IRSE International Technical Committee and is also a Liveryman of the Worshipful Company of Information Technologists. A chartered engineer, Clive has presented many technical papers over the past 30 years and his wide experience has allowed him to write on a wide range of topics for Rail Engineer since 2007.
Previous article
Next article


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.