From the 1840s mining activity developed in the ridge of terrain southwest of
Cinderford and traffic was being lost to the South Wales Railway's
Forest of Dean branch, even though the dock facility at Bullo, to which that line led, was inferior to that at Lydney. Accordingly, the board planned a new line,
the Mineral Loop, connecting to many of the new pitheads. It would be formed by extending the mainline from Wimberry Junction, near Cannop, to , near Cinderford; with the loop running from there to reconnect with the mainline at
Tufts Junction, near
Whitecroft. The new line would require a level crossing where it met the broad gauge branch of the
Forest of Dean Central Railway at
New Fancy and a tunnel at
Moseley Green. Initially, it would follow a fairly moderate descent, before a 1-in-40 drop down to Tufts Junction. The authorising act was the '''''' (
32 & 33 Vict. c. cxxxvii), receiving
royal assent on the 26 July. Construction of the Mineral Loop started in September 1870, but in February 1871 the Great Western Railway, as successor to the South Wales Railway, informed the S&WR of their intention to
convert the gauge of their mainline to standard gauge. The S&WR decided to lay the Mineral Loop in standard gauge and provide
mixed gauge on the existing broad gauge mainline from Tufts Junction down to Lydney Junction. In fact, the futility of retaining broad gauge on the mainline was realised, and it was converted to standard gauge on the weekend of 11–12 May 1872. On 22 April 1872 traffic started on the lower part of the Mineral Loop, below Crump Meadow, and it was substantially complete at the middle of May. The five broad gauge engines were converted to standard gauge, and a further engine was ordered from
Avonside Engine Company. By mid June 1872 the mainline was extended from Wimberry Junction to Drybrook Road, but in February 1873 the junction at
Bilson was still awaited. In fact commissioning of the junction at Bilson was delayed until 15 September 1873, and the through east–west iron ore traffic which had been foreseen was not started until November 1875, probably due to difficulties in agreeing rates. ==The Lydbrook Line==