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No-Signalman Token Remote (NSTR) is a signalling system developed in the 1980s to reduce running costs on lightly used single track railways in rural areas. Its full title is "No-Signalman Token on Single Lines with Remote Crossing Loops". NSTR is a less expensive alternative to Radio Electronic Token Block (RETB) but is less flexible in operation. It does however have the advantage of not requiring special equipment on board the trains. A long single track railway with intermediate crossing loops can be brought under the supervision of one signal box. Safety over the single line sections is governed by electric token instruments. At the remote crossing loops, the token instruments are operated by the traincrew. Withdrawal of a token requires the permission of the signalman, who therefore has a role in regulating traffic movements. The signalman maintains a visual reminder of the trains' positions using magnetic labels on a steel-backed diagram.
The first line to be converted to NSTR working was the Central Wales Line between Pantyffynnon and Craven Arms, in 1986. This line is under the control of the signalman at Pantyffynnon signal box. Although NSTR was subsequently introduced on various other lines, it has mainly been confined to the former Western Region, one notable exception being its provision on the Whitby line in 1989, controlled from Nunthorpe signal box.
Figure 1 shows the infrastructure at an NSTR remote crossing loop.
|Fig. 1: NSTR Remote Crossing Loop Layout.|
Train-operated (hydro-pneumatic) points are provided at both ends of the remote crossing loops. 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. Train-operated points have no facing point lock. A points indicator, usually in the form of an elevated position light signal, 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. An "end of section" notice board is placed at the entrance to each loop in the normal direction of travel. At either end of the loops are stop boards indicating the start of section and instructing drivers to obtain the relevant token.
A connection into a siding, where provided, is worked from a ground frame released by the appropriate section token. The sidings do not have the facility for trains to 'shut in'.
|Fig. 2: Token instrument hut.|
The basic principles of the Electric Token Block system apply equally to NSTR. A quantity of identical metal tokens are held in instruments at the extremities of every section. Electrical locking ensures that only one token for any section can be withdrawn at any time. The tokens are engraved with the name of the section that they apply to. Custody of a token is the driver's authority to occupy the named section. Adjacent token sections have key tokens of different configurations to prevent a token being inserted into the wrong instrument. The tokens are coloured according to their configuration.
A locked hut is located on each platform, close to the stop board at the entrance to the section ahead. Each hut contains two key token instruments. One is for receiving tokens from the section in rear and one is for issuing tokens for the section in advance. Tokens are normally surrendered into the 'receiving' instrument and withdrawn from the 'issuing' instrument. Consequently, the instruments have to be regularly balanced by maintenance staff who will remove excess tokens from the receiving instrument and transfer them to the issuing instrument on the opposite platform. Magazine balancers are provided for that purpose.
On arrival of a train into a crossing loop, the driver leaves the driving cab and unlocks the instrument hut. Having first checked that the train has arrived into the loop complete, he surrenders the key token for the previous section by inserting it into the 'receiving' instrument. It then becomes possible for another train to obtain a token for that section. Before withdrawing a token for the forward section from the 'issuing' instrument, verbal permission to do so is obtained from the signalman by telephone. Once in possession of the correct token for the section ahead, the driver can return to the train and drive past the stop board.
Beyond the train-operated loop points at the next remote crossing loop, the train will encounter a notice board worded "End of section - Proceed if platform line clear". This is a reminder to the driver that the line ahead might be occupied by another train. Due to the absence of continuous communication, drivers are unable to inform the signalman when their train has vacated a loop.
Train Protection & Warning System (TPWS) train stop system (TSS) loops have been fitted at NSTR stop boards. TPWS status indicators in the form of blue lights (one for each section) are provided inside the instrument huts. These indicators allow the train driver to confirm that the TPWS is working correctly. Normally a steady light is displayed, indicating that the associated TPWS is energised. When a token is withdrawn, the blue light applicable to that section will start to flash, indicating that the TPWS loops are de-energised. The loops remain de-energised for a set period of time, normally five minutes. If the light is out, there is a fault either with the TPWS equipment or with the indicator. The driver will report the fault to the signalman.
The train-operated points on most NSTR lines have been converted to conventional power operation. This change required the provision of points indicators at the stop boards applicable to trains departing a crossing loop in the usual direction. TPWS lineside status indicators have also been provided at these stop boards, in addition to the TPWS indicators inside the instrument huts. Stop boards applicable to trains departing a loop in the opposite direction are not provided with indicators but have the additional lettering "Check points before proceeding". The first crossing loop to be converted was Tenby, in December 2009.