What are Narrow Gauge Railways?
What is Narrow Gauge? Track gauge is the distance between the inner edges of the rails. In the UK, anything less than the (standard gauge) 4ft 8½in (1435mm) of the main line railways is considered 'narrow'. There was little standardisation of less than standard track gauge in the UK, although most Irish narrow gauge lines adopted the 3ft gauge. In the UK, there were lines of many different gauges from 3ft 6in or greater via 3ft, 2ft 6in, 1ft 11¾in, down to 18in and below. There is some debate about the smallest gauge for a 'proper' railway, before it become classified as 'miniature'. A fairly common view is that 18in is the minimum, although there are exceptions and there is a strong case for 15in!
A narrow gauge was generally chosen to reduce cost when building a railway, particularly where space was an issue or the terrain meant that building to standard gauge standards of curvature and gradient would be impractical or prohibitively costly. A significant consequence of this was that, in many parts of the world, main line railways were constructed on track gauges significantly less than standard. This remains so today and some prime examples are in South Africa, New Zealand, parts of Australia (Western Australia and Queensland) and Japan, where a gauge of 3ft 6in (1067mm, also known as 'Cape Gauge') is used.
It should be noted that the size of locomotives, carriages, and wagons is not directly related to the track gauge and, indeed, many overseas railways using the 3ft 6in (1067mm) gauge have larger locomotives and rolling stock than UK main line railways. The same is true on narrower gauges: A South African Railways 2ft gauge locomotive, such as the NGG16 class as used on the Welsh Highland Railway, dwarfs the locomotives used on UK narrow gauge lines and some of the smaller standard gauge locomotives. The adjacent diagram shows the size of the rolling stock on a number of railways from the British Isles, and includes a note of their track gauges.
For a quick summary of the narrow gauge beyond the UK, please click on this link.
Other 'gauges' that you will see referred to are Structure Gauge and Loading Gauge. The Loading Gauge is the maximum cross-sectional profile of rolling stock, including loads, able to run on a railway. Structure Gauge is the minimum cross-sectional profile of structures along the track (such as bridges, tunnels and platforms) in order to accommodate the rolling stock.
In the UK, most railways that were built as narrow gauge were feeder lines (linking e.g. quarries to ports or main line railheads) such as the Ffestiniog Railway or provided rail services to sparsely populated rural communities such as the Lynton & Barnstaple or the Welshpool & Llanfair. Others were purely industrial, working within factories or quarrying complexes. Later on, tourist oriented narrow gauge (and miniature) railways started to emerge, such as the Groudle Glen on the Isle of Man. This pattern was repeated overseas, in in some cases using British engineers and supplied with locomotives and rolling stock by British manufacturers. Temporary narrow gauge railways also played a significant role in the logistics systems on both sides of the front lines in World War I. Some former industrial and rural lines have been preserved, and are now purely tourist in nature.