Union Pacific Railroad 'Challenger' class 4-6-6-4

Built by The American Locomotive Company 1936-1944.

CYLINDERS (4) 21 ins x 32 ins (533mm x 813mm)
COUPLED WHEEL DIAMETER 69 ins (1,753mm)
TOTAL HEATING SURFACE
(including superheater) 6,383 square feet (593m2)
GRATE AREA 132 square feet (12.3m2)
BOILER PRESSURE 280 PSI (1,930.5kpa)
TRACTIVE EFFORT 97,400 lbs (44,180kg)
TOTAL WEIGHT 486 tons (493.8 tonnes)
LENGTH OVERALL 121 feet 11 ins (37,160mm)

In 1884 French engineer Anatole Mallet had an idea for the creation of a new type of locomotive. Having gained considerable knowledge of the virtues of compounding when applied to existing two cylinder types, his proposal called for a four cylinder machine driving two sets of six wheels. Before a successful design could materialize certain pitfalls needed to be addressed, namely weight distribution over the axles to gain maximum available adhesion coupled with the ability of the two units to act independently so as to allow the longer than normal contraption to negotiate bends on the existing trackwork. This would be achieved by incorporating a hinged device and sliding support into the frame of the front unit, the rear of which could be attached to the front of the rigid rear frame, thus allowing the long boiler to swing across the front unit as the locomotive entered a curve. It was also decided to have the primary high-pressure cylinders drive the rear unit before exhausting through flexible pipe work into the larger secondary low pressure cylinders mounted in the front unit. This arrangement became known as the Mallet and proved to be one of the more successful types of articulated locomotives.

The first American Mallet articulated was Baltimore and Ohio RR number 2400, an 0-6-6-0 compound, others followed on various railroads but it was not until 1918 that the Union Pacific RR introduced the first of seventy compound 2-8-8-0 Mallets. In 1924 the Chesapeake and Ohio RR decided to order its first articulateds, these were also 2-8-8-0’s but because a restricted loading gauge prevented the use of the enormous low pressure cylinders it was decided to opt for a simple expansion design incorporating the use of four smaller high pressure cylinders fed by flexible pipe work developed to carry high pressure steam. These new locomotives could do everything that their more ponderous compound cousins could but at higher speeds, eventually their success became widely known as other roads began to acquire them and the simple Mallet became the ultimate American freight locomotive.
In 1936 Union Pacific decided to join the procession and ordered forty simple expansion 4-6-6-4’s. They proved to be so successful that another sixty-five 'Challengers' as they were known were acquired to help move the wartime traffic. These new engines had a larger grate area 108 square feet (10m2) to 132 square feet (12.3m2) and Boiler Pressure up from 255 PSI (1,758kpa) to 280 PSI (1,930.5kpa); they also incorporated a fourteen-wheel centipede tender rather than the smaller twelve wheel tenders of the earlier locos. In 1945 five of the new engines were converted to burn oil fuel, were painted two-tone grey and had large smoke deflectors fitted for passenger train use.

It must have been an awesome spectacle to witness one of these goliaths rushing a twenty-coach passenger train across the desert at 70 mph. Number 3895 is preserved in working order.