In Britain the Great Western Railway pioneered broad gauge from 1838 with a gauge of 7ft 0.25in (2140mm), and retained this gauge into the 1890s. Many countries have broad gauge railways. Ireland and some parts of Australia have a gauge of 1600mm (5ft 3in). Russia and the other former Soviet Republics use a 1520mm (originally 1524mm) gauge while Finland continues to use the 1524mm gauge inherited from Imperial Russia (the two standards are close enough to allow full interoperability between Finland and Russia). The Baltic States have received funding from the European Union for rebuilding their railways to the standard gauge. Portugal and the Spanish Renfe system use a gauge of 1668mm. In India a gauge of 1676mm (5ft 6in) is widespread. This is also used by the Bay Area Rapid Transit system of San Francisco, California.
While Russia and Spain chose broad gauge to make railborne invasion by its enemies that much more difficult, most non-standard broad gauges get in the way of interoperability of railway networks. On the GWR, the 2140mm gauge was supposed to allow for high speed, but the company had difficulty with locomotive design in the early years (which threw away much of their advantage), and rapid advances in permanent way and suspension technology saw standard gauge speeds approach broad gauge speeds within a decade or two in any case. On the 1600mm and 1676mm gauges, the extra width allowed for bigger inside cylinders and greater power, a problem solvable by outside cylinders on standard gauge. On BART, the wider gauge is supposed to prevent lightweight trains getting blown over by the wind.
Where trains encounter a different gauge, such as at the Spanish-French border or the Russian-Chinese one, the traditional solution has always been transshipment - transferring passengers and freight to cars on the other system. This is obviously far from optimal, and a number of more efficient schemes have been devised. One common one is to build cars to the smaller of the two systems' loading gauges with trucks (bogies, in British parlance)that are easily removed and replaced, with switching of the trucks at an interchange location on the border. This takes a few minutes per car, but is quicker than transshipment. A more modern and sophisticated method is to have multigauge trucks whose wheels can be moved inward and outward. Normally they are locked in place, but special equipment at the border unlocks the wheels and pushes them inward or outward to the new gauge, relocking the wheels when done. This can be done as the train moves slowly over special equipment.