In 1978 a large section of the overhead signalling wire route on the Far North Line collapsed, due to the weight of snow and the age/condition of the wires, particularly the wires north of Tain. An alternative type of signalling, called "Working by Pilotman", was then used, which involved a Pilot-Man going into the cab of the train instead of a token. This system remained in place for an extended period, as the cost of repairing or replacing the wire route was deemed excessive at the time. The R&D Dept at Derby was tasked with finding a solution to the problem and the solution they came up with was a radio system that allowed the token instruments at each end of a section to be operated without a wire route, a bit like Bluetooth.
A further development came along later, which became known as Radio Electric Token Block (RETB) and which allowed an extended length of railway to be operated by only one signaller at a centre, in Dingwall for example. An instrument in the driver's cab, known as a Cab Display Unit (CDU), connected to the radio and meant that the driver acted as the second signaller. A token was displayed on the CDU and when the driver confirmed it was on display, the signaller who issued it then authorised the driver to proceed. The perceived advantage of this was a huge saving in employing signalling staff at the stations along the line.
After a shaky start that perceived saving was not all it had promised to be. This was because the system was designed for low density use, but the P'way (or maintenance) Dept got mobile units and so, instead of requiring lookout staff etc, they took an engineers' token, which protected them at work and blocked the line to prevent trains entering the section. As the train service got busier, the demand on the Network increased and so the signalling centre was working incessantly, with demands for token exchanges being prioritised.
The question of savings is dubious, because of the high cost of radio network licensing, the difficulty of accessing hilltop sites to do repairs in winter, shared sites which require agreement to access, the use of bespoke parts not common throughout the railway industry and padlocks often being found frozen when a driver needs access to equipment. The public lost a lot of the services to which they had previously had access at the stations, such as being able to purchase tickets, get train information and use waiting rooms and toilets.
The ideal solution would be an update of the system, this would go a long way to solving some of the issues. From Inverness to Tain, "Track Circuit Block" signalling could be used, which would not require drivers' participation in signalling. With this system, trains could follow each other much more closely and the signaller would have control of operating the points, giving greater flexibility, rather than hydraulic control points which are very time-consuming if the driver has to clamp or reverse them. This would free up the RETB to operate the remainder of the network and create a more manageable system for the Far North Line.