installation of the Train Protection & Warning System. A train stop loop is provided at each stop board, and is normally activated (so that any train attempting to pass it will be immediately brought to a halt). When the signaller issues a token for a train to enter a section, the TPWS loop at the appropriate board is deactivated, so allowing the train to proceed. Indication of the state of the TPWS is provided by a blue light mounted below the stop board. This shows a steady blue light when the TPWS is activated, and a flashing blue light when it is deactivated. History The genesis of the system was on the Far North Line, a long, remote single-track line between Inverness, Wick and Thurso in Scotland. This line was controlled by traditional electric token instruments at each station, but in January 1978 the signal telegraph pole route was brought down by bad weather over more than forty miles of track. The simplest, cheapest and quickest way of restoring the links between the instruments was found to be by radio: each machine was fitted with an external controller containing a unique microprocessor code so that the effect of a dedicated link to the machine at the other end of its section was maintained. The manual issue of the tokens continued as before. With the feasibility of using radio to effect the interlocking of single line token instruments demonstrated, and the additional benefit of voice communication between the signaller and the drivers noted, it was but a short step of invention to moving the instruments from staffed signal boxes to the cabs of trains. The line selected for the trial was another remote and lightly used Scottish line: the old Highland Railway route from Dingwall westwards to Kyle of Lochalsh. The contract was placed with Westinghouse of Chippenham, Wiltshire, and the system was brought into use on 28 October 1984, with the control equipment situated at Dingwall. Over the next four years, control was transferred to Inverness and the Wick and Thurso line was included in the scheme. A new control centre was brought into use at Banavie for the West Highland Line from Helensburgh Upper to Fort William and Mallaig, and from Crianlarich to Oban. The system was also used on two other British rural lines: the East Suffolk Line, where the control centre was at Saxmundham, and the Cambrian