The
leading wheel or
leading axle of a
steam locomotiveA steam locomotive is a locomotive powered by steam. The term usually refers to its use on railways, but can also refer to a "road locomotive" such as a traction engine or steamroller....
is an unpowered
wheelA wheel is a circular device that is capable of rotating on its axis, facilitating movement or transportation whilst supporting a load , or performing labour in machines. Common examples are found in transport applications. A wheel, together with an axle overcomes friction by facilitating motion by...
or
axleAn axle is a central shaft for a rotating wheel or gear. In some cases the axle may be fixed in position with a bearing or bushing sitting inside the hole in the wheel or gear to allow the wheel or gear to rotate around the axle. In other cases the wheel or gear may be fixed to the axle, with...
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 or locomotive, or sprung as in the suspension of a caterpillar tracked...
"). 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.
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The
leading wheel or
leading axle of a
steam locomotiveA steam locomotive is a locomotive powered by steam. The term usually refers to its use on railways, but can also refer to a "road locomotive" such as a traction engine or steamroller....
is an unpowered
wheelA wheel is a circular device that is capable of rotating on its axis, facilitating movement or transportation whilst supporting a load , or performing labour in machines. Common examples are found in transport applications. A wheel, together with an axle overcomes friction by facilitating motion by...
or
axleAn axle is a central shaft for a rotating wheel or gear. In some cases the axle may be fixed in position with a bearing or bushing sitting inside the hole in the wheel or gear to allow the wheel or gear to rotate around the axle. In other cases the wheel or gear may be fixed to the axle, with...
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 or locomotive, or sprung as in the suspension of a caterpillar tracked...
"). 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 arrangement of the wheel assemblies of a locomotive by type, position, and connections. There are serveral notations used to describe wheel arrangements, the use of which varies by country...
, his locomotive would be classified as a
4-2-0In the Whyte notation, a 4-2-0 is a railroad steam locomotive that consists of a 4-wheel leading truck followed by a single driving axle. This type of locomotive, often called a Jervis type, was common on American railroads from the 1830s through the 1850s.Other equivalent classifications are:
: 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 were 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 0-4-4In Whyte notation, an 0-4-4 is a railroad steam locomotive that has four coupled driving wheels followed by four trailing wheels, with no leading wheels. This type is often called in the USA a Forney, after Matthias N...
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 three years later...
. Other designers, however, persisted with the practice and the famous 0-4-2In Whyte notation, a 0-4-2 is a railroad steam locomotive that has four coupled driving wheels followed by two trailing wheels, with no leading wheels. 0-4-2 locomotives are typically tank engines, which is noted by adding a T to the end, 0-4-2T...
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 truck) 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 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.