Standard gauge: why most railways run on 1,435 mm

Gauge is the distance between the inner faces of the two running rails. It sounds like a detail that anyone could have settled once and for all, yet railways around the world use at least six common gauges, and the one that carries the most track is an odd-looking figure: 4 ft 8½ in. Where it came from, and why it survived, is a story about early habits becoming a rule.

The number, worked through

The imperial figure converts exactly as you would expect. 4 ft 8½ in is 4 × 12 + 8.5 = 56.5 in, and one inch is 25.4 mm:

56.5 × 25.4 = 1,435.1 mm

which is why the metric figure is quoted as 1,435 mm. That is the standard gauge, sometimes called Stephenson gauge. Roughly half of the world's railway track uses it, including most of Europe, North America and China.

Where 4 ft 8½ in came from

The gauge came from the coal mines of north-east England. Before steam locomotives ran on public lines, collieries used horse-drawn wagonways to carry coal down to a river or a staithe. Their track was laid to suit the local wagons, and in that region it was about 4 ft 8 in. George Stephenson started his career at such collieries. When he built locomotives, and then the Stockton and Darlington and Liverpool and Manchester railways, he used the gauge he already knew, and it is generally said the extra half inch was added to give the wheels a little play on curves.

Stephenson's lines were the first to prove that steam railways worked as a business, and they were copied. Engines, wagons and track were built to the same dimensions, so every new line that adopted them could exchange vehicles with the others. That was worth far more than any claimed advantage of the figure itself.

Brunel's broad gauge

Isambard Kingdom Brunel thought the standard was too narrow. For the Great Western Railway he chose 7 ft ¼ in, which is 84.25 in:

84.25 × 25.4 = 2,139.95 mm, or about 2,140 mm

That is 2,140 − 1,435 = 705 mm wider than standard, nearly 49% more (705 ÷ 1,435 = 0.49). A wider track allowed larger, steadier carriages and, as Brunel argued, higher speeds. His lines did run well. But a broad-gauge line could not exchange wagons with a narrow one. Where the two met, as at Gloucester, every passenger and every parcel had to change trains, and goods had to be unloaded and reloaded.

Parliament looked into the problem and settled it with the Gauge Act of 1846, which required new passenger railways in Great Britain to be built to 4 ft 8½ in. By then the narrow gauge already had far more route miles. The Great Western went on running broad gauge for decades, adding a third rail for mixed running, and the last broad-gauge track was converted in 1892. For forty-six years after the Act (1892 − 1846), the standard was the law for new lines without being the whole network.

The other gauges

Standard gauge won in Britain and in most of the places that copied British practice early. Elsewhere, countries chose differently for their own reasons, and several of those choices became national networks that are still in use.

• Russian, 1,520 mm. Used across Russia and the countries that were part of its railway system. It is 85 mm wider than standard.
• Iberian, 1,668 mm. Spain and Portugal. It is 233 mm wider than standard.
• Indian, 1,676 mm. That is 5 ft 6 in, since 66 × 25.4 = 1,676.4. It is 241 mm wider than standard.
• Cape gauge, 1,067 mm. That is 3 ft 6 in, since 42 × 25.4 = 1,066.8. It is 368 mm narrower than standard, and is common in southern Africa.
• Metre gauge, 1,000 mm. It is 435 mm narrower than standard.

Put side by side, the spread is wide. Cape gauge is about a quarter narrower than standard (368 ÷ 1,435 = 0.256), and metre gauge about 30% narrower (435 ÷ 1,435 = 0.303). The widest of the five, Indian gauge, is 241 ÷ 1,435 = 16.8% wider.

Why a mismatch costs so much

A train cannot run from one gauge to another, because the wheels of a standard vehicle will not sit on rails 85 mm further apart. Where networks meet, the usual answers are to transfer the load, to lift the vehicle and change its bogies, or to use axles that adjust their width. Each costs time at every crossing. That is the force that made Britain's argument over gauge so heated in the 1840s, and it is the same force that keeps national gauges in place today: changing one means altering every kilometre of track, every wheelset and every bridge and platform that was built around it.

Gauge also sits within a wider set of choices. It sets how wide the vehicles can be, how tight a curve is comfortable and how much a bridge must carry, which ties it to the ideas in our post on railway bridges. Narrower lines are cheaper to build in difficult country, where curves and cuttings can be smaller, and that is part of why narrow gauges spread through places with mountains. The heavy freight that justifies wider ones is covered in how freight chains work.

What to check

Converting 56.5 in gives 1,435.1 mm. Converting 84.25 in gives about 2,140 mm, which is 705 mm more. The Gauge Act is 1846 and the last broad-gauge conversion 1892. Gauge is always measured between the inner faces of the rails, so a source that measures from rail centres will give a larger figure.