1,435 mm between the inner faces of the two rails. It is the most widely used gauge, covering most of Europe, North America, the Middle East, and China.
Reference
The gap between the two rails
Track gauge is the distance between the inner faces of the two rails. Standard, narrow, and broad gauge each cover different parts of the world, for different reasons.
The width most of the world uses
At 1,435 mm between the rails, standard gauge is the most widely used width. It goes back to 1822 and the first public steam railway, then spread across most of Europe, the Middle East, North America, large parts of Africa, and China. Passenger, regional, long-distance, freight, and high-speed lines use it.
Why the number is 1,435 mm
The width is older than the steam railway. Early English coal tramroads used it so horse carts could keep running on the new rails without a change. The same spacing is widely linked to Roman chariot wheels, wide enough for two horses side by side.
Britain fixed 1,435 mm as its standard in 1846. In Austria today, about 5,200 of 5,575 km run on standard gauge. The other 370-odd kilometres are narrow gauge.
Tighter than 1,435 mm
Narrow gauge is anything under standard. The usual ones are metre gauge at 1,000 mm and 760 mm. It is used in mountains and difficult ground, because a tighter gauge takes sharp curves and steep grades better than standard gauge. Mountain and heritage lines in the Alps are the usual example.
Wider than 1,435 mm
Broad gauge is anything over 1,435 mm. The common one is Russian gauge at 1,520 mm, the second-longest rail network in the world after standard gauge. A wider gauge generally carries a heavier, more stable load, which is why it appears on long heavy-freight lines. The Trans-Siberian is the best-known example.
Why the widths still do not match
Most differences come from before any international agreement. Each region built its own railway to the width the builder wanted. In more than a few cases the mismatch was on purpose, so a rival army or a competing operator could not run its wagons on the line.
Where two widths meet, that is a break of gauge. The cargo moves onto wagons built for the new width, or the same wagons have their wheelsets exchanged or adjusted. Variable-gauge running gear has made that faster. It is still an extra step, and it still adds time when a route crosses the boundary.
New widths are unlikely from here. The practical trend is to extend the gauges already in place. Standard gauge in particular is growing quickly across East Asia.
FAQ
Questions that come up on a rail route
Narrow gauge is anything under 1,435 mm, mainly metre gauge and 760 mm, and suits tight curves and steep ground. Broad gauge is anything over 1,435 mm, most commonly Russian gauge at 1,520 mm, and suits heavy freight over long, mostly flat distances.
Most networks were built on their own, before any international standard existed. Terrain, cost, and sometimes a deliberate military or economic choice pushed different regions toward different widths.
The cargo moves onto wagons built for the new gauge, or the same wagons have their wheelsets exchanged or adjusted at the border. Either way, it is an extra step that adds time to the route.
Open the wagon types
The wagon page lists covered, open, flat, and tank wagons, with capacity, tare, and volume.