Lehigh Valley class J-55½

The Lehigh Valley had the ability to source large quantities of smaller fragments of anthracite coal as residues from the processing at less than a dollar a ton. In order to be able to use these as fuel for steam locomotives, the camelback design was used with a very wide firebox, which included a large grate area. A larger batch of ten-wheelers was produced from 1904, which were suitable for use with fast freight trains in the lowlands and heavy local trains.

The locomotives were designed so heavy that the axle load was just suitable for high speeds. The large firebox was a slightly modified Wootten type and did not have a combustion chamber. The load on the individual driving axles was distributed more rear-heavy to get better traction when starting. The initial wheel diameter of 68.5 inches represented a good compromise between higher speeds and a tractive effort of more than 30,000 pounds. 

There were some changes during production, for example the wheel diameter was increased to 69 inches. While the boiler initially had 378 tubes, this number was reduced to 363 from 1907. With the exception of Baldwin's last delivery, all locomotives had Stephenson-type valve gear. Of the last Baldwin locomotives, eight had Walschaert valve gear and two had Baker-Pilliod valve gear.