-wheel engine. The
desire to operate longer trains and the need for engines of greater
traction to overcome the steep grades of American roads called for
coupled driving wheels and machines of greater weight than the 4--2--0.
After the introduction of the 4--4--0, the single-axle engine received
little attention in this country except for light service or such
special tasks as inspection or dummy engines.

[Illustration: Figure 12.--THE "PIONEER" IN CARLISLE, PA., 1901. (_Photo
courtesy of Thomas Norrell._)]

There was, however, a renewed interest in "singles" in the early 1850's
because of W. B. Adams' experiments with light passenger locomotives in
England. In 1850 Adams built a light single-axle tank locomotive for the
Eastern Counties Railway which proved very economical for light
passenger traffic. It was such a success that considerable interest in
light locomotives was generated in this country as well as in England.
Nearly 100 single-axle locomotives were built in the United States
between about 1845-1870. These engines were built by nearly every
well-known maker, from Hinkley in Boston to the Vulcan Foundry in San
Francisco. Danforth Cooke & Co. of Paterson built a standard pattern
4--2--4 used by many roads. One of these, the _C. P. Huntington_,
survives to the present time.

The following paragraphs describe the mechanical details of the
_Pioneer_ as it appears on exhibition in the Smithsonian Institution's
new Museum of History and Technology.

BOILER

The boiler is the most important and costly part of a steam locomotive,
representing one-fourth to one-third of the total cost. A poorly built
or designed boiler will produce a poor locomotive no matter how well
made the remainder of mechanism. The boiler of the _Pioneer_ is of the
wagon-top, crownbar, fire-tube style and is made of a 5/16-inch thick,
wrought-iron plate. The barrel is very small, in keeping with the size
of the engine, being only 27 inches in diameter. While some readers may
believe this to be an extremely early example of a wagon-top boiler, we
should remember that most New England builders produced few locomotives
with the Bury (dome) boiler and that the chief advocates of this later
style were the Philadelphia builders. By the early 1850's the Bury
boiler passed out of favor entirely and the wagon top became the
standard type of boiler with all builders in this country.

Sixty-three iron tubes, 1-7/8 inches by 85 inches long are us