Why Is the Track That Width? An Accident That Became Permanent

The distance of four feet eight and a half inches between the inner faces of the rails came from the wagonways of north east England, where horse-drawn wagons carried coal on wooden and later iron rails. George Stephenson had worked on those lines and used the same figure for the Stockton and Darlington and then the Liverpool and Manchester, adding a half inch to an existing four feet eight to give the wheels a little more freedom. Because those were the first successful main lines and because Stephenson and his engineers built many that followed, the figure spread by precedent rather than by any calculation of what was best.

Other widths were tried and several were technically better:

• •Brunel built the Great Western at over seven feet
• •That gave a steadier ride, more space and higher potential speeds
• •It also cost far more in earthworks, tunnels and bridges
• •Narrower gauges were cheaper and suited mountains and colonies
• •Wider gauges were adopted in Russia, Spain, India and Ireland
• •Several countries adopted more than one and regretted it

The case is a standard illustration of how a technical standard becomes locked in regardless of merit. Once a network exists, every new line connecting to it must match, so each addition strengthens the case for the existing figure and weakens any alternative. Rolling stock, workshops, tooling, signalling and staff training all accumulate around it. A British commission settled the dispute in 1846 by legislating for the narrower figure on new lines, and the Great Western eventually converted its entire remaining broad gauge network over a single weekend in May 1892, with thousands of workers moving one rail inward along hundreds of miles of track.

Stating a gauge precisely turns out to need more care than a single figure suggests. The measurement is taken between the inner faces of the rails and at a specified depth below the top of the rail, because rail heads are rounded and a measurement at the very top would differ. Track is deliberately widened slightly on tight curves, since a rigid wheelset cannot follow a sharp bend at the nominal width without binding. Wheels are also coned rather than cylindrical, which lets a wheelset centre itself and steer gently through curves without flange contact, and that self-centring behaviour is one of the more elegant pieces of mechanical design in ordinary use.

Where networks of different widths meet, the consequences are permanent and expensive. Every through passenger must change trains and every wagon must be unloaded and reloaded, or lifted onto different bogies, or run on adjustable axles that are complex and costly. Spain and France, and Russia and its western neighbours, all deal with this daily, and it has repeatedly had military significance, since an invading army cannot use captured rolling stock. Australia inherited three different widths from separate colonies and spent most of the twentieth century and a great deal of money building standard gauge links between its major cities.

The common figure came from north east English coal wagonways by way of Stephenson, who added a half inch for clearance, and it spread by precedent rather than by any assessment of what worked best. Brunel's much wider gauge rode better and cost more, and was converted away in a single weekend in 1892. Where widths meet, everything must be transferred, which has cost Australia most of a century.