Ideal gauge

Ideal gauge

What constitutes an Ideal gauge depends on the purpose.

Engineers have shown that a narrow gauge is less than ideal: despite usually offering cheaper construction, a smaller gauge restricts speeds due to a reduced load stability. Broader gauges are theoretically more stable at speed and allow larger, wider, heavier loads. According to Isambard Kingdom Brunel's studies the optimum gauge for a rail system (and the one he originally used on his Great Western Railway) is 7 ft (2100 mm).

There has been much controversy about what constitutes the "ideal gauge". From a design point of view, a train can travel faster around a given radius of track if the gauge is wider, as the centre of gravity of the train is further displaced from the wheels, which in turn lowers the angle between the wheel's lower contact surface to the centre of gravity, and horizontal. Given that one can tailor either the track radius for train speed, or the train speed for track radius, gauge in some cases may not be as important as interoperability.

There are many examples of high speed and high mass applications on narrow gauges throughout the world, suggesting that gauge is less important than the original supporters of either broad gauge or narrower gauges held it to be:

* The heaviest trains in the world run on standard gauge track in Australia, North America and Mauritania. Gauge is not the limiting factor in running heavier trains.
* The fastest conventional trains in the world also run on standard gauge in Japan and Europe, where speeds over 300 km/h are attained.
* Very heavy trains run on the narrow gauge of RailGauge|42 in Queensland (Australia) and South Africa, on track as strong as heavy standard gauge track. This narrow gauge does not seem to materially affect the weight of trains that can be run on it.
* Fairly fast trains (160 km/h) can run on RailGauge|42 track, as can be seen in Japan and Queensland.
* It is possible to build a light standard gauge line about as cheaply as a narrow gauge line.
* It is possible to build a narrow gauge line to as heavy-duty a standard as a standard gauge line.
* Loading gauge, structure gauge, axle load, compatibility of couplings, continuous brakes, electrification systems, railway signal systems, radio systems and rules and regulations are also important.

With the benefit of hindsight, little was gained by building railway systems too narrow (down to about RailGauge|36) or too broad (up to about 7 ft (2100 mm)) gauges, and this was at the cost of limited interoperability. For an example of the difficulties of interoperability see the ramsey car transfer apparatus and the variable gauge axles used to transfer cars between different gauges of track.

Only in gauges of less than RailGauge|36 can a railway be built significantly more cheaply than is possible with standard gauge, and only then in mountainous terrain, or where a low capacity line is required, or with industrial railways where through running is not required.

It can be argued therefore, that the original uniform gauge adopted by Stephenson in 1830 can serve most of the tasks performed by gauges from 3 to 7 ft (900 to 2100 mm), albeit with a narrow gauge of about RailGauge|2ft for cane tramways, underground mine, mountain, construction, temporary and military railways, plus children's railways.

As the advantages of interchange of equipment between lines became clear, so did standardization of gauge become attractive. Where these advantages are not compelling, use of non-standard gauges continue today.

Sharper curves

Narrow gauge rolling stock tends to be smaller in all directions, so that they can cope with sharper curves. Broad and standard gauge rolling stock may have problems with the same sharp curves because:
* wheel base of carriages and wheel base of bogies is too long.
* Couplers cannot cope with very sharp curves, especially the British and continental European style of buffers, hooks and chains.
* Brake hoses cannot cope or disconnect with very sharp curves.

One might also add that if a too heavy train is pulled around a sharp curve, intermediate wagons may be pulled off the rails and cause a derailment.

For example, the sharpest curve on the RailGauge|3'6" gauge Queensland Railways is convert|200|ft, while the sharpest curve on the RailGauge|ussg New South Wales Railways is convert|330|ft.

Experience on the narrow gauge Toronto and Nipissing Railway suggests that 4- and 6-wheel wagons should be avoided and bogie wagons substituted.

Steam locomotives have problems with sharp curves because they may need many driving wheels to spread their weight which lengthens the wheelbase. Eventually flexible driving wheels such as on a Garratt locomotive were devise to tackle this problem. Having two small locomotives instead of one large one is not really a solution as this requires two crew instead of one. The Australian Standard Garratt had flangeless leading driving wheels to cope with sharp curves, but these proved to be derailment-prone.

A possible solution to the sharp curve problem is to build cheaply to begin with, to get the railway open; and, should traffic increase, expect deviations to ease these sharp curves, for example Cameroon. A really intelligent design will plan a cheaper and nasty short-term route with the long-term deviation planned at the same time so as to share expensive items such as viaducts and tunnels. This is easier said than done, as shown with the Cascade Tunnel which proved to be too steep. Queensland Railways built many of its original timber viaducts 20 m off the final alignment, so that a replacement steel bridge would be completely straight.


Wind can and does blow trains over on occasion, and the wider the gauge the better. However, this problem is rare, and with weather forecasts and warning devices, precautions can be taken. Monsoon winds were a factor in the choice of Broad Gauge in India, and for the lightweight BART trains in San Francisco. A train was famously blown over on a narrow gauge railway in Ireland. The narrow gauge trains of the island of Newfoundland, when stranded in severe winter weather, were once chained to the rails to prevent overturning. A double stack container train on the standard gauge railway was suspected of having had a few cars blown over during a storm near Tarcoola. [ARHS Railway Digest May 2008, p18] The second Tay Bridge is fitted with a device to warn of excessive wind speed.


ee also

* Narrow gauge railway#Advantages of narrow gauge
* Narrow gauge railway#Disadvantages of narrow gauge
* Rail gauge in India discusses reasons for choosing specific gauges

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