The story of Union Pacific's Challenger 4-6-6-4 steam locomotives from development to their final years. Illustrated with B&W photographs and extensive text.

Contents:
The First Challengers,
Union Pacific Influence,
Passenger Assigned,
Modifications 3900-3939 Early Series,
Operations 3900-3939 Early Series,
Jabelmann Power,
Extra 3950 West,
War-Time Duplicates,
Last Steam Design,
Modifications Later Series,
Operations Later Series.