Ideal gauge

Rail gauge
Broad gauge
Standard gauge
Scotch gauge
Narrow gauge
Dual gauge
Break-of-gauge
Rail tracks
Tramway track
List of Rail Gauges

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 3 ft 6 in (1,067 mm) 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 3 ft 6 in (1,067 mm) 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 3 (914 )) 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 3 (914 ) 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 2 (610 ) 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 3 ft 6 in (1,067 mm) gauge Queensland Railways is 200 feet ( ), while the sharpest curve on the 4 ft 8½ in (1,435 mm) New South Wales Railways is 330 feet ( ).

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 swiveling driving wheels such as on a Garratt locomotive were devised 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

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. The second Tay Bridge is fitted with a device to warn of excessive wind speed.

Deployment

The largest connected systems are those built to Standard gauge, Russian gauge and Indian gauge; each allowing through-passage for trains as far as one particular network extends.

Standard gauge is used for both the heaviest trains (running in Australia, North America and Mauritania) and for the fastest conventional trains 300+ km/h (Europe, Japan and Korea).

Specialist cases

In special applications, it can be advantageous to choose a gauge to geographical conditions or pre-existing local standards—in exchange, for forfeiting direct compatibility with general railway networks and the ability to buy off-the-shelf rolling stock.

Narrower gauges

Narrow gauge railways tend to offer cheaper construction and allow tighter radius curves. Infrastructure costs for bridges, right-of-way and tunnels are lower when less space is required. Narrow gauge railways are often constructed in mountainous areas.

Narrow gauge railways offer less lateral stability. With careful planning, narrow gauge systems can approach the specifications or wider systems. On 3 ft 6 in (1,067 mm) track, trains run at up to 160 km/h in Japan and Queensland, Australia. Very heavy mineral and coal trains run on 3 ft 6 in (1,067 mm) track in South Africa and Queensland, Australia, albeit with easy curves.

Articulated Garratt and Fairlie locomotives can be used to cope with sharper curves. The common specific uses of narrow gauge of use a gauge around 2 (610 ) for cane tramways, underground mines, mountain conditions, construction, temporary and military railways and pleasure miniature railways.

Wider gauges

Wider gauges above the median gauges require more gentle curves. Engineer Isambard Kingdom Brunel researched and concluded that the optimum gauge for a his railway system would be around 7 feet (2,133.6 mm). In theory the wider gauge offered increased stability at speed and would have allow larger, wider, heavier loads.

Brunel's Great Western Railway was later initially constructed to 7 feet (2,133.6 mm). Samuel W. Johnson, Chief Mechanical Engineer of the Midland Railway (1873–1904) stated:

"My ideal gauge for a railway is 5 feet 3 inches [1600 mm]. How many of the difficulties experienced by locomotive superintendents and mechanical engineers would have been avoided, had the 4 ft. 8½ ins. gauge been superseded years ago by the 5 ft. 3 ins. gauge. The crowding of machinery into the confined space between the frames limits the boiler diameter when the wheels are large, cramps the firebox width, and unduly reduces the dimensions of crank bearing surfaces and webs. All these obstacles would have disappeared, and many things which are now so difficult would have been easy of accomplishment, had the gauge been made somewhat broader".

Some of these problems were eased when inside cylinders were changed to outside cylinders, and wheel arrangements without trailing wheels such as 2-6-0 changed to arrangements such as 2-6-2 with trailing axles (which allowed wide fireboxes).

Johnson also failed to notice that 4' 8½" and 5' 3" are so similar that they do not easily allow dual gauge with heavy rails, such as happened a century later in Victoria. A lesson here is not to introduce needlessly new gauges with only small differences from existing gauges. With 3' 6" already pioneered in the British Empire in Queensland, why does India introduce metre gauge which is then exported to Africa, creating potential breaks of gauge there. Why did British India have both 2' 0" and 2' 6"?

Compatibility

For railway compatibility and to provide the opportunity for through-running, trains and infrastructure have to conform to more requirements than just sharing a common gauge.

These requirements include electrification systems, compatibility of couplings, brakes, signalling and possibly common radio systems. Rolling-stock and cargo must match both sytems' rules and regulations for loading gauge, structure gauge, axle load.

See also: Variable gauge axles and Ramsey car transfer apparatus

Planning to upgrade

Where building a straight railway would be too expensive, it is possible to build a winding contour following route first for less cost. Should traffic prove successful or warrant an increase, the curves can straightened and the distance shortened with new infrastructure and earth works. This method has been used in for railway construction in Cameroon.

A good railway planning design will plan a cheap or quick to construct route but plan for a long-term deviations at the same time. Queensland Railways built many of its original timber viaducts 20 metres off from the ideal final alignment, so that parallel replacement steel bridges would give a completely straight route.

See 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
• Heaviest trains
• Longest trains
• Minimum railway curve radius