The Union Pacific Railroad bought 45 4-8-4s in three orders from the American Locomotive Company.

The first order of 20 locomotives (road numbers 800 through 819) was delivered in 1937. These were designated Class FEF-1 and were designed by UP's Motive Power Group to have 77" drivers, 24.5 x 32 cylinders, a boiler pressure of 300 psi, and a weight of 465,000 lbs resulting in a tractive effort of 63,611 pounds (63,800 nominal).

The second order of 15 locomotives (road numbers 820 through 834) was delivered in 1939. These locomotives were designated as Class FEF-2. They were designed to have 80" drivers, 25 x 32 cylinders, a boiler pressure of 300 psi, and a weight of 483,000 lbs giving each a tractive effort of 63,750 pounds (63,800 nominal).

The last group of 10, Class FEF-3 were delivered in 1944 and were assigned road numbers 835 through 844. These FEFs (Four-Eight-Four) were designated Class FEF-3 and were very similar to the Class FEF-2's.

Comparing operating costs per mile in 1946:

	NYC Niagara	U.P. #800-844
Fuel	$0.410	$0.400
Water	$0.031	$0.031
Lubrication	$0.011	$0.011
Maintenance	$0.356	$0.330
Other supplies	$0.005	$0.005
Enginehouse	$0.100	$0.100
Crew wages	$0.1944	$0.1944
Vacation allow.	$0.0058	$0.0058
Social security	$0.0175	$0.0175
Fixed charges	$0.09	$0.09
Total Cost/mile	$1.22	$1.19

The UP 800 Four-Eight-Fours saved UP $300,000/year in costs. The purchase price for these engines was $150,000 in 1944 dollars. The 800s ran about 14,000 miles per month. Today these engines would cost around $10-11 per mile. The 800s cast-steel locomotive frame which replaced many separate parts by one single casting helped maintenance. Information provided by Dan McNaughton.

There are four surviving UP 4-8-4s: number 814 in Council Bluffs, IA; number 833 which was recently moved from Pioneer Park in Salt Lake City, UT to the Ogden Union Station Museum in Ogden, UT; number 838 at UP engine house in Cheyenne, WY and number 844, which is operational, also at UP engine house in Cheyenne, WY.