Gauge conversion train at Grange Court
Although the broad gauge survived until 1892, the decision to bring it to an end was made in the early 1860s. By 1866 there were 20 places where a change of gauge was required at junctions, causing much complaint and delay, particularly in the transhipment of goods between wagons. A petition from a large group of influential South Wales businessmen was made in the mid-1860s for a standard gauge rail to be provided on the South Wales Railway. A proposal was approved by the Great Western Railway (GWR) shareholders in early 1866 to convert the Hereford line to standard gauge and provide a mixed gauge line between Gloucester and Cardiff.
The necessary powers for the GWR to convert the gauge on all its lines was obtained in the Great Western Act of 1866. However an economic crisis in late 1866 resulted in the original plan not being carried out. In August 1868, the shareholders approved a revised plan which called for an end to broad gauge traffic on the Birmingham line north of Oxford and for the Hereford branch to be converted to standard gauge (as proposed in the 1866 plan). Additionally the little used broad gauge rail was also to be removed from the Metropolitan Railway in London.
For other reasons, however, the first lines to be converted were the section between Whitland and Carmarthen Bridge (where the gauge was mixed to prevent a competing line from being built to connect with the London & North Western Railway at Llandovery) and, in October 1868, the short Aylesbury branch was converted to standard gauge, so as to avoid expenditure on mixing the gauge at Aylesbury station for the Aylesbury & Buckingham Railway.
The last broad gauge train north of Oxford ran in March 1869 and in the following month the broad gauge rail was removed from the Basingstoke branch. The Hereford branch was converted in Aug 1869 and the exercise proved a useful guide on how to proceed with future conversions.
The line between Maidenhead to Oxford via Wycombe was converted in August 1870 and shortly thereafter, all London to Reading local passenger trains ran using standard gauge locomotives and rolling stock.
In 1871 a much more ambitious plan was approved by the GWR shareholders. This authorised the conversion to standard gauge of all lines from Swindon via Gloucester to South Wales. Also a standard gauge rail was to be added to the section of main line between Didcot and Swindon, enabling standard gauge trains to run between London and south Wales. The plan required around 500 miles of track and sidings to be converted and needed extensive planning. The Didcot to Swindon section was mixed in stages by February 1872. On 30 April 1872, the Up (London bound) line was closed from Gloucester westwards and a reduced single line service operated on the Down line with crossing points at Haverfordwest, Whitland, St Clears, Carmarthen Junction, Llanelly, Landore, Neath, Bridgend, Llantrissant, Cardiff, Newport, Portskewett and Lydney. Some services were temporarily diverted via other railway's lines in the district.
Conversion of the Up line was completed on 12 May and all remaining broad gauge stock moved to Swindon. The last broad gauge train in South Wales ran on Saturday 11 May 1872. As Swindon could not accommodate all the unwanted rolling stock, the Down line between Newbury and Hungerford was converted into a temporary siding. The 1,682 wagons owned privately were returned to their owners, or, in some cases, dumped on the side of the line near their owner's premises if the owner lacked sufficient storage space.
Standard gauge services using the Up line began on 12 May, by which time all the connecting single line branches had been converted to standard gauge. Conversion of the Down line was completed on 22 May and normal services resumed on 23 May 1872 using the standard gauge. Work then began on converting the Swindon to Gloucester and Cheltenham section, which followed the same procedure as used in South Wales, with crossing points at Stroud and Kemble. The Up line was ready by 26 May and the last broad gauge train left Cheltenham and Gloucester on 26 May. The Cirencester branch was closed between 23 May and 27 May, while conversion was carried out. Conversion of the Down line was completed on 28 May and normal services resumed on 29 May 1872. The broad gauge rails on the mixed gauge Bristol & Gloucester line (then owned by the Midland Railway) and the Vale of Neath line were also abandoned and gradually removed over the following months.
On 26 November 1872 the Didcot to Oxford line, along with the Abingdon branch, was converted to standard gauge. The conversion also saw the closure of the original Oxford station (by then only used for goods traffic). This meant that by the end of 1872, apart from the short Henley and Farringdon branches, the broad gauge had been abolished north of the original Great Western Railway main line between London and Bristol.
Proposals had been made as early as 1864 to add a standard gauge rail to the Salisbury branch, but no action was taken until 1874, when conversion was planned for all the lines in the district. The main line was mixed between Swindon and Thingley Junction in early June 1874 and by 4 July the lines to Weymouth and Salisbury had been converted to standard gauge, along with all the connecting branches, as well as the Reading to Holt Junction via Hungerford line.
The final broad gauge section of the London to Bristol main line (Thingley Junction to Bathampton) was converted to mixed gauge in April 1875, completing a direct standard gauge route between London and Bristol. All local traffic was soon changed to standard gauge rolling stock and sidings altered to suit. By early 1876, the broad gauge was only used for through traffic to and from London from/to points west of Bristol. However, no sooner had the Great Western Railway rid itself of its own broad gauge lines, than it acquired the Bristol & Exeter Railway and South Devon Railway, and as a consequence, control of the Cornwall Railway and West Cornwall Railway - all of which were wholly or predominantly broad gauge railways.