The 'American type' locomotive was, by far, the most popular and numerous form of locomotive in North America during the 19th century. Thousands were manufactured from the 1840s into the early 20th century.
The American type is characterized by 4 'guiding' or 'pilot' wheels up front, mounted in a swiveling bogie or 'truck' to guide the locomotive safely through curves and over rough track, together with 4 larger 'driving wheels' to propel the engine down the track.
Furthermore, the 4 driving wheels have an 'equalized' spring suspension, so that as the driving wheels on each side of the locomotive rock differentially up-and-down over uneven track, the weight borne by each of the wheels stays very close to equal. The 'equalizer' on Jupiter is the long, longitudinal leaf spring that is pivoted on a fulcrum on each side of the locomotive; this pivoted spring joins the suspensions of the two driving wheels on each side. As a result of this clever and limber arrangement, rails are not overstressed by varying weight on each wheel as the engine rolls along over uneven track, and the equal weight on each driving wheel keeps traction steady and strong.
This American innovation in locomotive chassis design was utterly vital, both to the operating success and the economic success of early U.S. railroads, whose track was cheaply and quickly built.
American railroad entrepreneurs and investors simply could not afford the very expensive, elaborately laid, and beautifully aligned track typical in Britain - where steam railroads originated. Unless locomotives in the U.S. could run dependably on our necessarily cheap and uneven tracks without frequent derailments, railroads could not have attracted either passengers or freight business. The 'American type' provided the dependability required.
The first American type, made in 1836 or 1837 by Henry Campbell, did not have an equalized driving-wheel suspension, and the lead truck did not swivel. But the locomotive, Gowan and Marx, of 1839 (photo of model, below) had equalizers and the swiveling truck. Partners Andrew Eastwick and Joseph Harrison developed the equalizer in 1837-38 and soon constructed the Gowan and Marx.
The engineer and inventor, John Jervis, designed the first locomotive to have a swiveling, 4-wheel lead truck. His successful Brother Jonathan of 1832 (originally called the Experiment) set the pattern for such a truck. Experiment (photo of model, below) tracked wonderfully in sharp curves and when rails were uneven, though it was short on power. Locomotives with the truck and a single pair of driving wheels proliferated.
(The Smithsonian's John Bull of 1831 benefitted from the new approach to locomotive agility. Originally a fairly typical English-made locomotive, with all wheels powered, John Bull apparently had trouble staying on the track at speed. Whatever the case, John Stevens and his mechanic Issac Dripps saw it necessary to modify the engine in 1832 or 1833 to incorporate a partly swiveling 'truck,' by adding a new pair of wheels out front, rigged to steer the locomotive. The so-called 'cowcatcher' was an afterthought.)
After 1839, the American type took on a grace and style that became a familiar icon of the 19th century.
The Phantom of 1857 (photo of model, below), designed and built by the then-celebrated William Mason, was such a locomotive. Mason locomotives were noted for their symmetry and clean lines. He did not want any of his products to look like a "cooking stove on wheels" [see J.H. White, Jr., History of the American Locomotive, 1979 ed., p. 384].
Special Note: The Smithsonian's Jupiter is definitely not the Jupiter that was one of the two locomotives present for the joining of the rails of the world's first trans-continental railroad, in northern Utah on May 10, 1869. Our Jupiter was not built until 7 years after that event. And our Jupiter is a different gauge (see Specific History, below).
Many 'American Type' locomotives were named Jupiter. Names such as that -- and Vulcan, Atlas, Hero, Hercules, Comet, etc. (there was at least one Manager on a New England railroad) -- were chosen to inspire awe and wonder.