A 4-4-6-4, in the
Whyte notation
Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It was devised by Frederick Methvan Whyte, and came into use in the early twentieth cen ...
for the classification of
steam locomotives by
wheel arrangement
In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and c ...
, is one that has four
leading wheel
The leading wheel or leading axle or pilot wheel of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located on a leading truck. Leading wheels are used ...
s followed by four coupled
driving wheels, a second set of six coupled driving wheels and four
trailing wheel
On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle ( wheelset) located behind the driving wheels. The axle of the trailing wheels is usually located in a trailing truck. On some large locomotives, ...
s.
The
Pennsylvania Railroad's
Q2 class were the only locomotives ever to use this arrangement. These were
duplex locomotive
A 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. The concept was first used in France in 1 ...
s, in which both sets of driving wheels were mounted in a common, rigid
locomotive frame
A locomotive frame is the structure that forms the backbone of the railway locomotive, giving it strength and supporting the superstructure elements such as a cab, boiler or bodywork. The vast majority of locomotives have had a frame structure o ...
. This locomotive design was a further development of the highly successful
2-10-4. The
divided drive, or
duplex arrangement, allowed for higher speeds with less damage to the track.
Other equivalent classifications are:
UIC classification
The UIC classification of locomotive axle arrangements, sometimes known as the German classification''The Railway Data File''. Leicester: Silverdale, 2000. p. 52. . or German system,Kalla-Bishop P.M. & Greggio, Luciano, ''Steam Locomotives'', Cre ...
: 2BC2 (also known as German classification and
Italian classification
The UIC classification of locomotive axle arrangements, sometimes known as the German classification''The Railway Data File''. Leicester: Silverdale, 2000. p. 52. . or German system,Kalla-Bishop P.M. & Greggio, Luciano, ''Steam Locomotives'', Cre ...
)
French classification Under the French classification system for locomotive wheel arrangements, the system is slightly different for steam and electric/diesel vehicles.
Steam
The French system counts axles, rather than wheels. As with Whyte notation, a conventional r ...
: 2232
Turkish classification In the Turkish classification system for railway locomotives, the number of powered axles are followed by the total number of axles. It is identical to the Swiss system except that the latter places a slash between the two numbers.
Thus
0-6-0 beco ...
: 2435
Swiss classification
For more than a century, the Swiss locomotive, multiple unit, motor coach and railcar classification system, in either its original or updated forms, has been used to name and classify the rolling stock operated on the railways of Switzerland. ...
: 2/4+3/5 up to the early 1920s, later 5/9
References
46,4-4-6-4
{{Steam-loco-stub