A total of nineteen of these locomotives were built by the PRR's Altoona Works (now owned by Norfolk Southern) between 1881–1883.
These locomotives were equipped with 78-inch (1,981 mm) drivers and seven were later converted to 72-inch (1,829 mm) drivers and classified D6a. These engines were one of the first American 4-4-0s to have the firebox above, rather than between, the locomotive's frames.
This added about 8 inches to the possible width of the firebox, enabling a larger, easier to fire and more powerful locomotive; the maximum fire grate area increased to about 35 sq ft (3.25 m2) from the previous maximum of about 18 sq ft (1.67 m2).
The innovation was not wholly new, having been first seen on the Philadelphia and Reading Railroad's 1859-built Vera Cruz, designed by James Milholland of that road and built in their own shops; the Reading used this design until the invention of the Wootten firebox in 1877.
It was subsequently adopted by the Baldwin Locomotive Works in 1881 for six locomotives constructed for the Central of New Jersey; these were followed by the Pennsylvania Railroad locomotives, which garnered more attention for this design feature, in addition to having larger drivers than most previous 4-4-0s.
They were all scrapped by the mid-to-late-1900's, a few lasting to the 1910's, and having been superseded by newer designs, not to mention scrapped after more than 20 years in service, there are no existing D6s today.
- None of these engines were used during WWI, because they were being sold off for scrap.
- These engines were high-drivered (78") passenger locomotives and coal-fired, too, as indicated by the straight, capped stack. This elaborate decoration is now out of fashion, for many later locomotives on the road have a plain, thin-rimmed, stove-pipe for a stack.
- The reason they were scrapped its because of bigger locomotives when they came in the mid-to-late-1900's.
- As the competition for high-speed intercity rail service began to heat up in the last quarter of the 19 th Century, the Pennsylvania Railroad developed a new series of ultra-modern passenger locomotives.
- Originally classified as type “K”, the new anthracite-coal-burning engines boasted tall 78” drivers, steam-powered reversing gear and a Westinghouse brake system. To save time, their tenders were equipped with water scoops, allowing them to refill their tanks while on the move.
- The locomotive’s sleek new profile eliminated the protruding sand dome, excessive “gingerbread” decoration and boasted an elegant black and gold decoration scheme.
- Pulling the PRR’s newest coaches, series PD, between New York and Philadelphia, the D6 maintained a scheduled time requiring a speed of almost 50 mph, including stops over the 90 mile route.
- In the 2019 live-action film, "Dumbo", the Medici Bros. Circus engine No. 41 nicknamed "Casey Junior", is based on a D6, though it has lamps and metal bars on the smokebox that resemble a face, a diamond stack, a shorter 6-wheeled tender, and no running board. In addition, the engine is a wood-burner and does not have a water scoop under the tender. Later in the end the tender is relettered to Medici Family Circus, and the lamps and metal bars that resemble the face on the smokebox were removed.