In September 2019, my wife and I spent a week in the Forest of Dean. On one day, we visited the Dean Forest Railway at Norchard.  Around the site at Norchard are a number of permanent outdoor displays.
The featured image in this post shows three gauges that for a very short time were all in use on the trackbed of the Severn and Wye Railway through the Forest.
The original gauge was the track-gauge used by the Severn and Wye Tramroad. Rails were cast iron and each section was around 1 metre in length. They were held in place not by timber sleepers but by stone blocks placed on the line of the rails. The gauge (or spacing between the two rails) was 3ft 6ins. This gauge was used by many of the branch tramways in the forest.
When the Severn and Wye became a Railway rather than a Tramroad the standard gauge for the Great Western Railway was what we now call ‘broad-gauge’ – 7 ft 1⁄4 in (2,140 mm). The Severn and Wye was constructed to the same standards as the Great Western, so avoiding the need for transhipment facilities at railheads such as Lydney Junction.
The Great Western Railway lost the ‘gauge war’ in the UK. Standard-gauge became the gauge first used by George Stephenson – 4 ft 8 1⁄2 in (1,435 mm) gauge. After the decision was made that all future lines in the UK would be built to the standard-gauge, there was period of mixed-gauge operation (tracks were laid with three rails), the Great Western Railway did not complete the conversion of its network to standard gauge until 1892. 
All three of these gauges could be found on the formation of the Severn and Wye Railway until 1892.
However, these were not the only track gauges in use in the Forest. An example of a different gauge in use us provided by Mr Brain’s Tramway which linked Trafalgar Colliery and Drybrook to Bilson Sidings and Transhipment Wharfs. It had a gauge of 2 ft 7 1⁄2 in (800 mm). Brain chose this gauge for his tramway because it matched the gauge used underground within his collieries and so saved an additional transhipment cost at the pithead. The display outlining the use of tramways which is on show in the Dean Forest Railway Museum at Norchard.The display outlining the change of gauge which is on show in the Dean Forest Railway Museum at Norchard.
1. https://www.deanforestrailway.co.uk, accessed on 14th September 2019.
2. https://en.m.wikipedia.org/wiki/Standard-gauge_railway, accessed on 15th September 2019.
3. Ian Pope; Mr Brain’s Tramway; Archive No. 84, Lightmoor Press, Lydney, p2-32.