4-6-6-4

[1] A similar wheel arrangement exists for Garratt locomotives, on which both engine units swivel, but is referred to as 4-6-0+0-6-4.

Today, the only Challenger locomotives that survive were both owned by Union Pacific.

It was retired in January 2020 due to its poor mechanical condition[2] and subsequently donated to the Railroading Heritage of Midwest America, where it is now undergoing a second restoration.

[3][4][5] The second example, Union Pacific 3977 is on static display in Cody Park North Platte, Nebraska.

Though originally intended for freight service, many units could be found for leading passenger consists as well.