The 2-6-6-6 (in Whyte notation) is an articulated locomotive type with two leading wheels, two sets of six driving wheels and six trailing wheels. Only two classes of the 2-6-6-6 type were built. One was the "Allegheny" class, built by the Lima Locomotive Works. The name comes from the locomotive's first service with the Chesapeake and Ohio Railway beginning in 1941, where it was used to haul loaded coal trains over the Allegheny Mountains. The other was the "Blue Ridge" class for the Virginian Railway. These were some of the most powerful reciprocating steam locomotives ever built, at 7,500 hp (which was exceeded by only the Pennsylvania Railroad class Q2 in indicated horsepower), and one of the heaviest at 386 tons for the locomotive itself plus 215 tons for the loaded tender.
Other equivalent classifications are:
UIC classification: 1CC3 (also known as German classification and Italian classification)
French classification: 130+033
Turkish classification: 34+36
Swiss classification: 3/4+3/6
The UIC classification is refined to (1'C)C3' for simple articulated locomotives.