The
leading wheel or
leading axle of a
steam locomotiveA steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
is an unpowered
wheelA wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...
or
axleAn axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...
located in front of the driving wheels. The axle or axles of the leading wheels are normally located in a truck (or "
bogieA bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...
"). Leading wheels are used to help the locomotive negotiate curves and to support the front portion of the boiler.
Importantly, the leading bogie does not have simple rotational motion about a vertical pivot, as might first be thought. It must also be free to slip sideways to a small extent (otherwise the locomotive is unable to follow curves accurately – a point lost on the 19th century railway pioneers), and some kind of springing mechanism is normally included to control this movement and give a tendency to return to centre. The sliding bogie of this type was patented by
William AdamsWilliam Adams was the Locomotive Superintendent of the North London Railway from 1858 to 1873; the Great Eastern Railway from 1873 until 1878 and the London and South Western Railway from then until his retirement in 1895...
in 1865.
The first use of leading wheels is commonly attributed to
John B. JervisJohn Bloomfield Jervis was an American civil engineer. He was America's leading consulting engineer of the antebellum era . Jervis was a pioneer in the development of canals and railroads for the expanding United States...
who employed them in his 1832 design for a locomotive with four leading wheels and two
driving wheelOn a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...
s (a type that became known as the
Jervis). In the
WhyteThe Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal...
system of describing
locomotive wheel arrangementsIn rail transport, a wheel arrangement is a system of classifying the way in which wheels are distributed beneath a locomotive.. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country...
, his locomotive would be classified as a
4-2-0Under the Whyte notation for the classification of steam locomotives, 4-2-0 represents the wheel arrangement of four leading wheels on two axles, two powered and coupled driving wheels on one axle, and no trailing wheels...
: that is to say, it had four leading wheels, two
driving wheelOn a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...
s, and no
trailing wheelOn a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle located behind the driving wheels. The axle of the trailing wheels was usually located on a trailing truck...
s. In the
UIC classificationThe UIC classification of locomotive axle arrangements describes the wheel arrangement of locomotives, multiple units and trams. It is set out in the International Union of Railways "Leaflet 650 - Standard designation of axle arrangement on locomotives and multiple-unit sets". It is used in much...
system, which counts axles rather than wheels and uses letters to denote powered axles, the
Jervis would be classified 2-A.
Locomotives without leading trucks are generally regarded as unsuitable for high speed use. The British Railway Inspectorate condemned the practice in 1895, following an accident involving two
0-4-4Under the Whyte notation for the classification of steam locomotives, 0-4-4 represents the wheel arrangement of no leading wheels, four powered and coupled driving wheels on two axles, and four trailing wheels on two axles...
s at Doublebois, Cornwall, on the
Great Western RailwayThe Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...
. Other designers, however, persisted with the practice and the famous
0-4-2Under the Whyte notation for the classification of steam locomotives, 0-4-2 represents the wheel arrangement with no leading wheels, four powered and coupled driving wheels on two axles, and two trailing wheels on one axle...
Gladstone classThe London, Brighton and South Coast Railway B1 Class is a class of 0-4-2 express passenger steam locomotives, known from the name of the first, No. 214, as the "Gladstones".-History:...
passenger expresses of the
London, Brighton and South Coast RailwayThe London, Brighton and South Coast Railway was a railway company in the United Kingdom from 1846 to 1922. Its territory formed a rough triangle, with London at its apex, practically the whole coastline of Sussex as its base, and a large part of Surrey...
remained in trouble-free service until 1933. A single leading axle (known as a
pony truckA pony truck, in railway terminology, is a leading truck with only two wheels.Its invention is generally credited to Levi Bissell, who devised one in 1857 and patented it the following year. Hence the term Bissel bogie or axle is used in continental Europe...
) increases stability somewhat, while a four-wheel leading truck is almost essential for high-speed operation.
The highest number of leading wheels on a single locomotive is six as seen on the
6-2-0In the Whyte notation, a 6-2-0 is a railroad steam locomotive that has an unpowered three-axle leading truck followed by a single powered driving axle...
CramptonA Crampton locomotive is a type of steam locomotive designed by Thomas Russell Crampton and built by various firms from 1846. The main British builders were Tulk and Ley and Robert Stephenson and Company....
type and the
Pennsylvania RailroadThe Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....
's
6-4-4-6A 6-4-4-6 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, is one with six leading wheels, two sets of four driving wheels, and six trailing wheels.Other equivalent classifications are:...
S1The PRR S1 class steam locomotive was an experimental locomotive that was the largest rigid frame passenger locomotive ever built. The streamlined Art Deco styled shell of the locomotive was designed by Raymond Loewy....
duplex locomotiveA duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive...
and
6-8-6In Whyte notation, a 6-8-6 is a steam locomotive with:* six unpowered leading wheels arranged into a three-axle leading truck,* eight powered driving wheels, and* six unpowered trailing wheels arranged into a three-axle trailing truck....
S2The Pennsylvania Railroad's class S2 was a steam turbine locomotive. Only one prototype was built, #6200, delivered in 1944. The S2 was the sole example of the 6-8-6 wheel arrangement in the Whyte notation, featuring a six-wheel leading truck, eight driving wheels, and a six-wheel trailing truck...
steam turbine. Six-wheel leading trucks were not very popular. The Cramptons were built in the 1840s, but it wasn't until 1939 that the PRR used one on the S1.