Today an explosion in freight traffic means it is a vital part of an Anglo-Scottish link on which £60m is being spent to double its capacity.

Many signals along the 72-miles of scenic railway, including the 24-arch Ribblehead Viaduct built in the 1870s, were removed when BR stopped all freight trains in 1983 and decimated the passenger service. But now an overwhelming business case exists to restore the line's capacity.

Signal boxes at Settle station, Horton-in-Ribblesdale, Selside and Ribblehead were removed in the 1970s and 1980s and block sections – lengths of line on which only one train is allowed at a time to eliminate the risk of collisions – were lengthened in the interests of economy.

This built bottlenecks into the system. Once a train leaves Settle Junction northbound, nothing else can pass that point until the train has cleared Blea Moor – 14 miles to the north on the flank of Whernside, the highest of Yorkshire's Three Peaks. A heavy freight train can take 35 minutes to complete the 1 in 100 climb.

Even if it gets a good run at the notorious Long Drag, which challenged drivers and firemen in the days of steam, a freight train weighing 2,000 tonnes can have slowed to 15mph by the time it reaches the summit at a height of 1,150ft. Similarly, the growing number of coal trains travelling south from opencast mines in Ayrshire to fuel power stations at Ferrybridge, Eggborough and Drax face a tough, slow, 17-mile climb from Appleby to Ais Gill summit before they begin running down into Ribblesdale.

Since 2005, remaining signal boxes have been staffed for 24-hour operation but now eight new intermediate block signals are being installed to reduce the length of time that following trains have to wait for the one ahead to clear the section.

The aim is to reduce the delay to better than 15 minutes. The chairman of the Friends of the Settle-Carlisle Line, Mark Rand, says: "There will be knock-on benefits from improvements on the Settle and Carlisle as far away as West Yorkshire, Strathclyde and on the West Coast Main Line."

Among options evaluated by Network Rail was the installation of locomotive cab signalling, which is not yet available; full resignalling, which was dismissed as too costly; reinstatement of signal boxes and a tail lamp camera system to check that an unbroken train had passed a given point.

Intermediate blocks were chosen and Network Rail has already begun installing the first of them on the up (southbound) line at Lazonby, between Carlisle and Appleby. Others will be added at Ormside, Crosby Garrett, Mallerstang, Ais Gill and Horton-in-Ribblesdale.

On the down line many of the trains are empty wagons returning north, so intermediate blocks are not as cost effective. Only two are planned on the northbound line at Horton-in-Ribblesdale and Lazonby. The first new signals at Lazonby are expected to be commissioned this summer.

Mr Rand says the 30,000 people who opposed the closure of the Settle-Carlisle Line – reprieved by the Government in 1989 – have been vindicated by its 21st century growth in traffic. He is pleased that Network Rail's investment is also taking into account possible future uses.

He said: "A case in point is Horton Quarry, where stone might one day be taken out by rail. Various infrastructure scenarios are identified which could accommodate stone traffic."