The track gauge of a railway, the back-to-back distance between the inner edges of the rails, is one of the more critical dimensions of a rail system; systems with gauges that differ are fundamentally incompatible, incapable of interoperability. Certainly, track gauge is not the only parameter essential to such interoperability; loading gauge, the cross-sectional profile within which a rail vehicle must fit, is almost as critical, as are such matters as couplings, signalling systems, and much else. However, none are such a fundamental incompatibility as gauge.

Many different gauges are in use around the world. About 60% of the world's route miles are of the so-called Standard Gauge of 4' 8½"; the rest are either broader or narrower. There are many reasons for this variance worldwide. Moving a rail vehicle between gauges generally required the swapping of wheelsets or trucks, which became a well-practiced art in some cases of trains that had to reguarly undergo a gauge change. More recently, movable wheel systems have been developed, e.g. by the Spanish Talgo company.

A certain taxonomy of track gauges can be seen. While no formal system exists to definitively categorize them, the following is my own system which more-or-less fits in with those I've seen held by others. It also pretty much follows the practice of the builders of locomotives and railway equipment, who generally carried standard designs to fit any gauge within each category. In other words, a manufacturer, say Baldwin, would have a small narrow gauge locomotive suitable for 1'6"-2'6" lines, a larger narrow gauge design suitable for 2'6" - 3'6" lines, and so forth.

Under 1'6" (500mm): Miniature and Model
Few if any railway systems intended for work have been constructed to gauges this small. However, railways for tourists, entertainment rides, and other such applications are regularly built to gauges in this range. Model railways, obviously, also fall into this category. The division betwen miniature and model is not hard and fast, but one possible division is that miniature railways carry human passengers, while model ones cannot.
1'6"-2'6" (500-750mm): Small Narrow Gauge
Much cheaper to build and operate than a full-size railway system, but narrow-gauge railways in this category are just too small to replace a full-size system. They tend to be local in scope and limited in purpose. Many of these systems were built for a specific industry; mining, quarrying, farms, works railway systems, and the like. These include most of the Welsh narrow gauge lines (Ffestiniog, Talyllyn, etc.), which mostly served the slate mining industry. Some were built for general purpose service, but they tended not to last.
2'6"-3'6" (750mm-1000mm): Large Narrow Gauge
Large enough to be a serious proposition as a general-purpose railway system, large narrow gauge lines like these proliferated in areas where the cost of a standard-gauge system would have proved prohibitive. Often, the tighter turns possible in a narrow gauge system reduced the expensive civil engineering works required to an affordable level. These narrow gauge systems were especially popular in mountainous areas, good examples of which are the numerous Swiss narrow gauge systems, most of which still thrive today, and the Colorado narrow gauge lines in the USA. They were also popular in the mining and logging industries, where the tracklaying was required to be cheap and quick, but substantial load-carrying ability was required.
3'6"-4'8½" (1000-1435mm): Narrow Full Size
Gauges in this range were often adopted as the 'full-size gauge' in many regions of the world, particularly Africa and Australia. While narrower than the Standard Gauge, locomotives and cars could attain nearly the same dimensions as true Standard Gauge lines. Gauges in this range were acceptable choices as a full-size gauge.
4'8½" (1435mm): Standard Gauge
The standard track gauge of Europe and North America, and in addition many other areas around the world. While it became 'standard' only because of historical precedent, it certainly seems to be right on the 'sweet spot' between too wide and too narrow. Wider, and minimum radii become too great, and the cost of civil engineering works become intolerable; narrower, and the ability to carry heavy or large loads at speed becomes dangerously compromised through instability.
4'8½"-5'6" (1435-1700mm): Broad Full Size
Standard gauges wider than 4'8½" were adopted in many places, including Spain, Eastern Europe and much of Asia, and parts of Africa and Australia. Permitted trains somewhat larger than Standard Gauge, but in general trains on these railway systems are little larger than the average. Gauges in this range provide an acceptable choice.
Greater than 5'6" (1700mm): Broad Gauge
These gauges are practically extinct. While gauges narrower than the ideal range of somewhere between 3'6" and 5'6" have the temptation of cheaper construction, the same does not hold true for broad gauge. The biggest proponent of a broad gauge was the early Great Western Railway in England, and in particular its chief engineer, Isambard Kingdom Brunel. The GWR at first built to the imposing gauge of 7'0¼" (approx 2140mm), but incompatibility with the Standard Gauge everyone else built to doomed the idea. Other proponents of a broad gauge included some early American railroads (though few built wider than 6') and Hitler's Nazi Germany (which produced grandiose plans for super-wide trains, practically ocean liners of the rails).

Railway track and railroad carriage gauge transfer

Gauge is the distance measured between the inside edges of two rails, or the outside edges of two wheels. There must be a match between wheels and track. Changes in railway track and railway car gauges may happen when moving across country borders, within countries, and also in mountainous regions where narrower gauges are more common.

There are different ways to accommodate gauge changes in railway tracks and railway carriages.

Laying extra rails is a way to handle different railway car gauge widths.

Here's an example diagram of twin gauge tracks:

 I         I I 

Here's an example of triple gauge tracks:

 I I      I  I 

Another way to deal with differing railway gauges is to make adjustments to the rail cars so they fit different track gauges. Engines do not usually have that ability and are swapped out at the gauge change transfer point, but there are two ways to adjust railway cars to fit different track gauges. Both involve changing the width between railway car wheels.

The wheels on a rail car are attached at each end of the car, and are called "bogies".

Some rail cars have detachable bogies, allowing swapping different wheel gauges onto the rail car. The cars can either be lifted off the track, or else placed on a section of track that can be lowered, which allows the bogies to be removed and replaced with other bogies to handle another gauge. The location where this is done, and the act, are both called a "bogie exchange".

A less common solution to the mixed gauge scenario is adjustable bogies, which still require removal from the track. Adjustable bogies don't have to be removed from the cars because they have a mechanism to change the width between their wheels.

Most people are familiar, in general, with the idea of railroad tracks - parallel metal rails on which platforms (or cars) with matching wheels can travel. Anybody who has lived near them will also be familiar with the basic principles of their construction. They are usually laid on a raised bed or mound covered by gravel and supported by many perpendicular ties, made of wood - or more recently, specially engineered concrete1 - placed between the mound and the rail itself.

What many people outside of the vibrant but dwindling railroad enthusiast community are not aware of, however, is the oddly important microcosm of track gauges. Track gauge, very simply, is the distance between the inside edges of the two load bearing rails. Depending on the application, it also sometimes also refers to specifications regarding non-load bearing rails which may carry and return electrical current or perform other specific functions.

The most common gauge by far is known appropriately enough as standard gauge. Sometimes called Stephenson gauge, after its primary historical proponent, slightly more than 60% of the rail in the world2 is standard gauge, which is 4 ft 8 1⁄2 in, or 1,435 mm wide. First used as part of an early horse-rail system in British coal mines, the standard gauge was eventually made the legal standard for passenger-carrying rails by a UK Royal Commission in 19453, in a period known as the Gauge Wars.

There is a persistent urban legend regarding standard gauge, in circulation since at least 19374, that standard gauge was descended directly from specifications of Roman chariot wheel width. Further investigation into historical and archaeological evidence suggests that a fairly standard width of around 5 feet for horse-drawn carts was necessary mostly to be able to fit a horse within the width of the frame5. Indeed, there are remains of pre-Roman stone roads with chiseled ruts that fall within the same specifications, and even more evidence of Neolithic-age carts with similar dimensions6. This suggests a technological tradition totally separate from the needs of permanent wheel ruts, likely having to do with stability, strength of unimproved materials in certain applications, etc.

Other railroads gauges still in use are Indian, Iberian, Irish, two different Russian gauges (Imperial and Metric), Cape, and Metre gauges. They account for anywhere from less than one percent (for some of those not listed) of worldwide track length, to almost 10%, as with the second variant of Russian gauge2. The current Russian (Metric) track is almost entirely contained in one continuous system found in the former Soviet satellites.

When trains must transfer between two incompatible gauges, what is known as a rail gap or break-of-gauge, there are several solutions, none of which are particularly timely or cost effective. One option is to lift each car and change the bogies, or wheel sets; another is to transfer the cargo onto another train entirely. The process can be dangerous, time-consuming, and highly technical.

Gauge differences can have interesting results other than a few hours of delay for passengers. For example, the break-of-gauge between Northern and Confederate rail systems during the US Civil War lead almost directly to a scorched Earth policy on the part of Northern armies. It was reasoned that, since the Northern military would have to rebuild the rails entirely7 to use them anyway, it made sense to raze the Confederate rails as quickly as possible, since the Confederacy was highly dependent upon rail transport for logistical support8. The utter destruction of captured rails meant that a large portion of Confederate manpower and production was sapped in the continuous battle to rebuild track that was necessary to transport the most basic of supplies to front line troops, rather than engaging in other industry to support the war effort.

Similar situations have cropped up throughout history, and have been part of military planning for at least as long as railways have existed. One of the reasons that India and Pakistan have different rail specifications, besides a general lack of interest in modernization, is to prevent each other from using the rail systems in the event of cross-border war. Even the current "grand unification" of Indian railroads9 has been slated to use a gauge incompatible with Pakistan's.

There are of course other reasons for using other than standard gauge in a rail project; during the Westward expansion of the US, many mining company railroads were built with a 3 foot wide track in order to save time and money on materials and labor when building rail specifically to service gold mines. These rails often had to be carved out by hand through narrow and treacherous mountain passes, and the resources saved by laying narrower track made up for the relatively small penalties incurred by the narrower track. The steep (for the day's technology) and winding paths taken were such that a wider gauge would have allowed for only marginal increases in speed and load, and the cars were not expected at any point to trans-load to other track systems.

Modern examples of nonstandard special purpose gages are the 7 foot gauges used for some loading yard crane systems, wildly differing miniature railroads, triple and quadruple rail city trolley services, and proprietary factory floor loading or machine moving systems. As with the mining company trains, these are typically not intended to ever face a break-of-gauge, and the gauge is determined by the need for load bearing capacity and stability, as with the crane systems, vendor lock-in as with factory automation systems, or other concerns, typically practicalities or economic burden.

Part of Industry Quest 2011.

1. US DOT Federal Railroad Administration research summary, "Dynamic Wheel Load Detector Extends Life of Concrete Railroad Ties"
2. IHS, Jane's World Railways
3. United Kingdom of Great Britain and Ireland Royal Commission "Railway Regulation (Gauge) Act 1846"
4. Townsville Daily Bulletin, "STANDARD RAILWAY GAUGE"
5., Horse's Pass
6. Karel J. Hughes, "Persistent features from a palaeo-landscape: the ancient tracks of the Maltese Islands"
7., Confederate Railroad Map
9. The Hindu, "‘Grand alliance’ demands operationalisation of railway line"

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