As the DL&W hauled anthracite coal, Zerah Colburn tried to design a locomotive that could also use this as fuel. The slow combustion of this hard type of coal required a large, i.e. wide grate. To keep the center of gravity low, he positioned the grate behind the rear drivers and designed a long firebox with a combustion chamber that was between the rear drivers. A large dome could be found inside the cab that was located on top of the boiler. The No. 17 “Lehigh” was delivered in 1855, followed by five more.

As the New Jersey Locomotive Works made some changes inside the boiler and omitted the combustion chamber, the locomotives initially did not produce enough steam. Afterwards, the boilers were rebuilt into the form originally intended by Colburn. Another problem that was found were the large masses of the firebox and the cylinders, both hanging well outside of the wheelbase, what created a see-sawing motion. Although the camelback layout was later successfully used for anthracite-burning locomotives, these six locomotives were not seen as a great success and later at least No. 17 was rebuilt to a 4-6-0.