In Whyte notation

Whyte notation

The 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...

, a 6-8-6 is a steam locomotive

Steam locomotive

A 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...

 with:

• six (6) unpowered leading wheel
  Leading wheel
  The leading wheel or leading axle 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 in a truck...
  
  s arranged into a three-axle leading truck,
• eight (8) powered driving wheel
  Driving wheel
  On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...
  
  s, and
• six (6) unpowered trailing wheel
  Trailing wheel
  On 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 arranged into a three-axle trailing truck.

Other equivalent classifications are:

• UIC classification
  UIC classification
  The 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...
  
  : 3D3 (also known as German classification and Italian classification)
• French classification: 343
• Turkish classification
  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.Thus0-6-0 becomes 33...
  
  : 410
• Swiss classification: 4/10.

The S2

PRR S2

The 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 locomotive

Steam turbine locomotive

A steam turbine locomotive is a steam locomotive which transmits steam power to the wheels via a steam turbine. Numerous attempts at this type of locomotive were made, mostly without success...

, built for the Pennsylvania Railroad

Pennsylvania Railroad

The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....

, was the only one ever to use this 6-8-6 wheel arrangement.

The engine built in 1944, used a direct-drive steam turbine, which ensured a smooth uniform power flow (torque or tractive effort) at all speeds. As the locomotive did not use cylinders, there was no rail hammering

Hammer blow

Hammer blow, in rail terminology, refers to the vertical forces transferred to the track by the driving wheels of a steam locomotive and some diesel locomotives. The largest proportion of this is due to the unbalanced reciprocating motion, although the piston thrusts also contribute a portion to it...

 as with reciprocating engines, so that the wheels only required counter-balances for the coupling rods. Consequently, the wheel diameter was small at 68 inches (1727 mm). The turbines drove the two middle axles via a series of reduction gears, however high pressure steam hits the blades at speeds up to 2,000 "miles per hour" [sic] which in turn generated about 6,900 horsepower

Horsepower

Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...

 (5.1 megawatts). The output exceeded all conventional steam locomotives as well as diesels rated at 6000 hp, above 40 mi/h. At speeds less than 30 mi/h steam consumption was high, but above that speed, its steam consumption was well below normal locomotives. Turbine maintenance was a major problem and the engine only ran until 1949. Some of its impressive attributes included; boiler pressure 310 psi (2.1 MPa); grate area 120 ft² (11 m²), tractive force 70,500 lbf

Pound-force

The pound force is a unit of force in some systems of measurement including English engineering units and British gravitational units.- Definitions :...

 (314 kN); engine weight 589,920 lb (267,580 kg).(Staufer, pp. 240-242)