Track gauge
The track gauge is the distance between the inner faces of the two rails. A train can only run on track of its own gauge, so gauge choices made in the 19th century still shape railway networks today.
Standard gauge (1,435 mm, or 4 ft 8½ in) is used by roughly 55 % of the world's lines, including most of Europe, North America and China, as well as nearly all dedicated high-speed lines. Broad gauges are wider and narrow gauges are narrower. Where two gauges meet, passengers must change trains, bogies must be exchanged or variable-gauge trains, such as some Talgo designs, adjust their wheelsets while running slowly through a gauge changer.
| Gauge | Name | Main regions |
|---|---|---|
| 1,676 mm | Indian gauge | India, Pakistan, Argentina, Chile |
| 1,668 mm | Iberian gauge | Spain, Portugal |
| 1,600 mm | Irish gauge | Ireland, parts of Australia and Brazil |
| 1,520 mm | Russian gauge | Russia, former Soviet Union, Mongolia (Finland: 1,524 mm) |
| 1,435 mm | Standard gauge | Most of Europe, North America, China, high-speed lines |
| 1,067 mm | Cape gauge | Japan (conventional lines), South Africa, New Zealand, Indonesia |
| 1,000 mm | Metre gauge | Switzerland, Southeast Asia, East Africa, many tramways |
| 760 mm | Bosnian gauge | Austria, Balkans (mountain and forest railways) |
| 600 mm | Two-foot gauge | Industrial, military and heritage railways |
UIC axle arrangement
The UIC classification describes how the axles of a locomotive or multiple unit are arranged. Capital letters count consecutive powered axles (A = 1, B = 2, C = 3, D = 4) and numbers count unpowered axles. An apostrophe (') indicates that the axles are mounted in a bogie, and a lowercase o means that each powered axle has its own motor.
For example, Bo'Bo' is the most common layout of modern electric locomotives: two bogies, each with two individually powered axles. Co'Co' has two three-axle bogies and is typical of heavy freight locomotives. (A1A)(A1A) describes two bogies with three axles each, where only the outer axles are powered. On articulated trams, Bo'2'Bo' means two powered bogies with an unpowered bogie in between, and a plus sign separates units that are coupled together.
Steam locomotives can also be described in UIC notation, sometimes followed by letters for the steam system: 2'C1' h2 is a Pacific locomotive (two leading axles in a bogie, three coupled driving axles, one trailing axle) using superheated steam (h) and two cylinders.
Whyte notation for steam locomotives
In English-speaking countries, steam locomotives are classified with the Whyte notation, which counts wheels rather than axles: the number of leading wheels, then driving wheels, then trailing wheels. A 4-6-2 has four leading wheels, six coupled driving wheels and two trailing wheels. Articulated locomotives add a group, such as the 4-8-8-4 “Big Boy”. Many arrangements have names:
| Whyte | Name | Typical use |
|---|---|---|
| 0-6-0 | Six-coupled | Shunting and light freight |
| 4-4-0 | American | 19th-century passenger trains |
| 2-8-0 | Consolidation | Freight |
| 4-6-0 | Ten-wheeler | Mixed traffic |
| 4-6-2 | Pacific | Express passenger trains |
| 2-8-2 | Mikado | Freight |
| 4-8-4 | Northern | Heavy express and fast freight |
| 4-8-8-4 | Big Boy | Articulated heavy freight (Union Pacific) |
Electrification systems
Electric trains receive current from an overhead line (catenary) or a third rail. Several systems coexist around the world, which is why many locomotives and multiple units are built as multi-system vehicles:
600–750 V DC is standard for trams, metros and third-rail suburban networks. 1.5 kV DC is used in the Netherlands, southern France and on Japan's conventional lines. 3 kV DC equips Italy, Belgium, Poland, Spain's Iberian-gauge network and part of Russia. 15 kV 16.7 Hz AC is the system of Germany, Austria, Switzerland, Sweden and Norway. 25 kV 50 Hz AC is used in France, the United Kingdom, China, India and on most high-speed lines, while Japan's Shinkansen uses 25 kV at 50 or 60 Hz depending on the region.
Power output, tractive effort and top speed
Power output (kW or hp) is the rate at which a locomotive can deliver energy to its wheels. It determines the speed a train can hold on a gradient and how quickly it accelerates once moving. The continuous rating is the power the vehicle can sustain indefinitely, while short-term ratings may be higher.
Tractive effort (kN or lbf) is the pulling force at the wheel rim. Starting tractive effort, limited by adhesion between wheel and rail, decides the heaviest train a locomotive can start; this is why heavy-haul locomotives are both heavy and fitted with many powered axles. Axle load is the weight carried by each axle and determines which tracks and bridges a vehicle may use.
Transmission describes how engine power reaches the wheels. Diesel-electric locomotives use a generator and electric traction motors, diesel-hydraulic ones a torque converter, and diesel-mechanical ones a gearbox, like a car.
Types of rolling stock
A locomotive is a traction vehicle that hauls coaches or wagons. A multiple unit is a set of passenger cars with its own traction, either electric (EMU), diesel (DMU) or diesel-electric (DEMU). A railcar is a single self-propelled vehicle, and a control car (or driving trailer) is an unpowered car with a cab used for push-pull operation.
The formation of a multiple unit gives its number of cars and, often, the split between motor (M) and trailer (T) cars: “4 cars (2M2T)” means two powered and two unpowered cars. See all categories on our train types page.
Units and conversions
Our spec sheets use metric units and show imperial equivalents: 1 km/h = 0.621 mph, 1 kW = 1.341 hp, 1 kN = 224.8 lbf, 1 tonne = 2,204.6 lb and 1 m = 3.281 ft. Weights are given in metric tonnes (1,000 kg).