Around the world, planners constructing new railway lines must determine what rail gauge to adopt. Historically, many different gauges have been used for a range of reasons. Although this variation has lessened over time, there remain many locations where passenger and freight rail traffic move from one gauge to another. Several methods have been developed to make these operations as efficient as possible.

1. What exactly is the rail gauge?

The term “rail gauge” traditionally refers to the track gauge, which is the distance between the insides of the two rails. But there are other gauges as well—principally the structure gauge, which defines the vertical and horizontal clearances around the track to allow clear passage of the rolling stock. There have been an astonishing number of track gauges used over the years, but today most fall into six groups.

2. Why are there so many rail gauges?

The first railways developed from horse-drawn wagonways, which had a variety of gauges but were typically between 1200mm and 1500mm. The “standard gauge” of 1435mm was more or less the gauge of a wagonway used by the engineers (principally George Stephenson) of the earliest U.K. railways in the 1820s. But other engineers had their own theories, generally wanting to build broader gauges to provide more stability and capacity. Many of these variations were eventually converted during the following decades. Narrower gauges, which typically permit sharper curves than the standard gauge and are cheaper to construct, were subsequently introduced as lines were constructed in hillier terrain and as networks expanded into low-volume regions.

In some cases, gauges have been considered an important element of commercial and national security. Variations in gauge between neighboring networks limited diversion of traffic and interchange of rolling stock with adjacent networks. Some countries still consider a break-of-gauge an important factor in strengthening their defenses against invasion.

3. Which gauge is the best for efficient freight operations?

The general engineering consensus has always been that the most technically efficient gauge in normal terrain is somewhere between 1500mm and 1800mm—but the gain compared to the standard gauge of 1435mm is not overwhelming. In addition, most modern rolling stock outside Russia and India is built as standard gauge, and the cost of modifying the design to a different gauge is often substantial. Finally, for those rail systems which use cape and meter gauges, change to standard gauge is more often than not hardly justifiable on economic grounds. It entails loss of interoperability with neighboring railways and complete change in rolling stock. This is usually not underpinned by significant gains in freight volume.

4. How do railways solve the problem of transshipment at interchange points?

Freight often has to be physically transshipped, either manually or through bulk transshipment facilities. Many interchange points also have facilities for changing the bogies of the vehicles while the contents remain untouched, and for short distances a wagon of one gauge can be piggybacked onto a transporter wagon. But these arrangements cost money, delay freight, and are unreliable. Another common solution has been to avoid the problem by including both gauges in the same track. Dual gauge exists over significant distances of some main lines, and triple gauge can also be found.

5. To what gauge should new railway lines be constructed?

Where new lines are connecting with a network, the cost and inconvenience of transshipment is often so steep that there is every incentive to construct the same gauge as the network it is connecting to. Where the new line is standalone (as with high-speed lines in Japan and Spain as well as the Gautrain suburban line in South Africa), however, there are strong arguments for constructing a standard-gauge line.