Train detection

From Festipedia, hosted by the FR Heritage Group

Train detection is an integral part of signalling systems. The knowledge of the location of a train can be used to alert a signalman or level crossing keeper of the approach of a train, or to control automatic level crossings. Ultimately train detection is used in a signalling system as part of the interlocking between points and signals to prevent unauthorized or unsafe train movements.

In traditional signalling systems with mechanical operation of signals and points, there was initially great reliance on the signalman being able to see the location of trains when managing train movement. One of the most important responsibilities laid on the signalman was not to move facing points either under a train or when a train was approaching. Various mechanical and electrical means of detecting the location of a train have been devised, three of which are used on the Ffestiniog & Welsh Highland Railways.

Until the commissioning of Rhiw Goch signal box in 1975, the primary use was to indicate the approach of a train at level crossings and to replace colour light signals automatically to Danger at Minffordd, Penrhyn, Tan y Bwlch, and the electrically operated mechanical Up Advance signal at Harbour Station as the train went past the signal. This was achieved by means of a treadle, so this device will be considered first, followed by what are called locking or fouling bars as used at Rhiw Goch and Dduallt; finally the most common form of train detection, the track circuit, will be considered.


In the early days of the FR revival a home-made treadle was installed by Howard Wess below Quarry Lane Crossing (Lottie's); it sounded a bell in the crossing keeper's cottage. Presumably down trains were considered more predictable. It did not survive long, being ripped by a swinging wagon hook. A second version was built with flanges operating on a sponge-rubber strip and a burglar alarm type micro-switch but this did not last long either before the flanges and the damp made inroads on the rubber. By 1964 this had been replaced by very basic track circuits without insulated fishplates in Cemetery Cutting and above Weigh House Crossing – which was a rather amateurish installation. They were inclined to ring Lottie's bell when heavy rain improved the conductivity of FR rail joints.[1]

Silec Treadle – JMW sketch for FR S&T Reference Book 1969
Silec Treadle Rail Fixing BR – JMW sketch for FR S&T Reference Book 1969

In the c. 1970 phase of re-signalling the FR, the Signal Gang bought redundant Silec treadles from the Southern Region that had been displaced by the Hixon automatic level crossing upgrade programme. These were overhauled and used in the electrically operated signal schemes at Harbour, Minffordd, Penrhyn and Tan y Bwlch. The treadle detects the first wheel (flange) of a train, and for a moving train therefore detects when the last vehicle has passed. However, should the train stop over the treadle, with no flange in contact, then the treadle would return to the normal position. The use treadles are put to takes these operating characteristics into consideration.

At the intermediate stations the treadles replaced to Danger the colour light Home signals when a train passed the signal. At Harbour the new Up Advance signal was a motor-operated centrally balanced (somersault) signal which was replaced to Danger when a train departed across the Cob. Operation of this treadle by an incoming, Down, train caused an audible warning in the Booking Office and the Catering Dept. One treadle was also used in this period to sound an audible warning at Penrhyn level crossing of Down trains approaching, though being uni-directional it also rang for Up trains.

John Wagstaff (JMW) drew two diagrams for the technical reference book that he and David Josey developed for the FR S&T: Drawings SG-72-1 & 2. As they are drawn on foolscap size sheets they do not quite scan vertically onto an A4 sheet.

The Silec treadle features a trip arm depressed by a passing wheel flange. The trip arm is connected to electrical contacts used in circuits as part of the signalling or level crossing system. To reduce the impact of multiple train wheels depressing the trip arm, a shock absorber is provided and a dashpot is used so that the trip arm returns to its normal position slowly.

Figure 2 on sheet SG-72-1 shows how the treadle was bolted to a cantilevered bracket which was clamped to the underside of the running rail. This was for flat bottomed rail; for bull-head rail the fitting used two bolts through the web. This was sketched from the BR arrangement. On the FR the treadle took up most of the space between the tracks on the two foot, so we clamped two flat strips of metal across the track, thereby helping the p-way to keep their track in gauge – the usual considerateness that the S&T show their p-way colleagues! This could be done without the need for insulation as track circuits were not in use. To protect the treadles from hanging couplings etc. a heavy-duty steel sheet was placed over the treadle which was bolted to sleepers.

Treadles are now mainly used in the control of automatic level crossings, with both strike-in to initiate the crossing sequence and strike-out functions to end the sequence. In addition they are used to ‘announce’ the approach of trains at Harbour station and Penrhyn crossing and to reset the starting signals at Porthmadog and Tanygrisiau to danger after the passage of a train.

Depression or Fouling Bars[edit]

Fouling Bar showing the principle of the mounting and the electric contact from the FR S&T Reference Book 1969

Rhiw Goch was the first installation of what might be called a comprehensive signalling system on the FR since World War 1. It was recognized that the bottom end points were quite some distance from the signalbox at the top end of the loop. It was essential therefore to know the exact location of trains, or more precisely where they were not, in order to ensure that the trains were not on the points and were clear of what is known as the Fouling Point. The Fouling Point is important as it defines where there is clearance between trains on adjacent lines at a set of points.

When Rhiw Goch was installed ready for the 1975 High Summer season it was not possible to use track circuits, firstly because such a device was not available to meet FR requirements and secondly because there was insufficient power available at this remote location.

The track circuit came into common use on UK main line railways in the early years of the 20th century, although such a device had been used as early as 1872 in the USA. Prior to this a mechanical alternative was used in some locations to lock points in position when a train was passing over them. This was the depression or fouling bar, a long metal strip mounted on the inside of a rail so that it is depressed by the wheel flanges of a train. A series of springs return the bar to its normal position once the train moves on. In early installations the depression of a bar was used to lock the points mechanically in position, preventing the signalman from pulling the point lever and possibly derailing the passing train.

The installation at Rhiw Goch is quite possibly now unique in the UK as depression bars for interlocking purposes went out of fashion many decades ago. It is also unusual in that rather than using mechanical locking at the points, an electrical device is used to detect when a train is passing, this in turn electrically locks the point lever in the signal box. A total of five such depression bars are used, as illustrated in this extract from the Local Instructions: three at the bottom points and two at the top points.

Diagram of Rhiw Goch showing five depression bars

The loop at Dduallt was commissioned after Rhiw Goch. Unlike Rhiw Goch, Dduallt used track circuit detection, as described below. However, the bottom-end trap points had already been fitted with a mechanical depression bar, and this was retained. The depression bar was connected to the facing point lock rodding, so that pulling the lever to unlock the facing point lock would raise the bar. With a train preventing the depression bar from being raised, it was therefore not possible to unlock the facing point lock and move the points. This depression bar can be seen in picture iBase 2176 .

Track Circuits[edit]

The use of track circuits became an essential part of the automated signalling systems to be installed at Minffordd and Tan y Bwlch; however, conventional DC track circuits as used by British Rail with very low voltages presented a problem for the designers. The FR features many low axle load vehicles, particularly wagons, and the build-up of contamination such as rust due to the relatively low frequency of trains. This presented a problem for the signalling designers.

The solution came in the form of a type of track circuit developed specifically for application on the FR by Damian Poole (1947–1999). This took as its basis the Lucas track circuit that saw limited used on BR in the early 1950s. Damian took the basic idea of a higher voltage DC impulse signal fed into the rails that would cut through the contamination to ensure good electrical contact between rail and wheel, and at the same time work with low axle loads. At the time Damian worked in the Research and Development Department of GEC-General Signal at Trafford Park in Manchester, so in addition to being skilled in the needs of railway signalling he had access to others with relevant knowledge and laboratory facilities to design and build prototypes. It also has to be said that his boss, Ian Foster, was an FR member and supported Damian’s endeavours.

Damian's objective was to design a track circuit that could be built, tested and maintained by people not having specialised technical knowledge; he selected components that were readily available from a variety of suppliers (such as Radiospares [now RS Components Ltd.]). Having built prototypes, he carried out practical tests on the FR with slate wagons, to prove that the high voltage led to reliable operation, even on infrequently used sidings. He then constructed batches of these track circuits (and perhaps other FR members built them as well), and he could often be found in GEC's Trafford Park labs in the evenings, assembling and testing the equipment.

So was born the FR High Voltage DC Impulse Track Circuit that has become standard wherever train detection using track circuits is required. The initial installation was at Dduallt when the signalbox there was commissioned (between 1971 and early 1973; see Dduallt signalling circa 1970) (although the bottom-end trap points retained their mechanical fouling bar from when they were originally operated from a ground-frame).

It is believed that other heritage railways expressed interest in these track circuits, and that some units were actually sold (to the Bluebell, possibly? Citation needed).

In 1984, a consortium including GEC won the turnkey contract to build the original section of the Docklands Light Railway in London, and needed a low-cost track circuit for use on sidings within the railway's Poplar Depot. Damian suggested that his FR design could be adapted, and so a commercialised and repackaged version was designed, and some tens of units built and installed. GEC obtained the rights to use this design in return for a donation to the FR. The GEC version of these track circuits was only used on one further project shortly after DLR – a fully automatic railway to move tank-shaped targets at speed around an army firing range at Lulworth Camp in Dorset. There was some concern about whether the high voltage impulses between the rails would be injurious to cattle that passed over a level crossing on the access line from the depot, and the signalling on this section had to be powered down before the cattle were moved!

Damian's untimely death in 1999 resulted in his family donating his personal stock of electronic components, and test instruments including an oscilloscope, to the FR, at the suggestion of Bob Barnard (another FR member working in the same department). The closure the following year of the Manchester R&D department of GEC (by this time part of Alstom) and consequential abandonment of some little-used designs, led to Bob Barnard obtaining agreement for copies of the GEC drawings of the commercialised version of the track circuit to be passed to Martin Duncan at Boston Lodge for future reference. The installations at Minffordd and Tan-y-Bwlch have proved very successful, having been in service as an integral part of the automated signalling at these stations since 1987 (Tan y Bwlch) and 1989 (Minffordd).[2][3]

More recently this style of track circuit has been installed at Cae Pawb and Harbour station, the equipment having been manufactured ‘in house’. In both of these installations the track circuits are integral to the signalling systems.

See Also[edit]


  1. ^ Dan Wilson (1964) "Lottie's Treadle", Ffestiniog Railway Magazine, Issue 27, Winter 1964/65, page(s): 10 - 11
  2. ^ "S&T and Electrical", Ffestiniog Railway Magazine, Issue 118 (1987), page(s): 11
  3. ^ "Signalling, Telecommunications and Electrical", Ffestiniog Railway Magazine, Issue 125 (1989), page(s): 197