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.
References:
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. Snopes.com, Horse's Pass
6. Karel J. Hughes, "Persistent features from a palaeo-landscape: the ancient tracks of the Maltese Islands"
7. Americancivilwar.com, Confederate Railroad Map
8. Csa-railroads.com
9. The Hindu, "‘Grand alliance’ demands operationalisation of railway line"