By Lloyd E. Stagner and TRAINS Magazine

AT&SF Test 87312 indicated that the 3766 performed even better than calculated. Maximum drawbar horsepower was 4550 at 45-50 mph, dropping to 4450 at 60 mph, 4160 at 70 mph, 3720 at 80 mph, and 3120 at 90 mph. Indicated horsepower reached a maximum of 5450 at 60-65 mph and was in excess of 5300 at all speeds between 48 and 80 mph. The boiler evaporated a maximum of 82,000 pounds of water per hour. The test report concluded: "The general performance of Locomotive 3766 was satisfactory and demonstrated clearly on later runs the economy of high boiler pressure with limited cutoff which permitted development of high horsepower through a wide speed range. The trains handled during these runs, except San Bernardino to Summit, did not require capacity operation but a short period before the trains were on time."

After a few trips with the improved Nos. 3751 and 3753, it became obvious that their 73-inch drivers were not properly counter-balanced for 90-mph speeds, so a restriction of 77 mph was imposed; however, the other 12 engines' limit was raised to 70 mph. Management began a program in 1938 to improve the 3751's, making them suitable for 90 mph maximum speeds on the La Junta-Los Angeles engine district. No. 3753 was the first turned out of the Albuquerque shop on November 11, 1938, and No. 3762 followed on November 29.

These improvements included the application of new one-piece CSC engine bed with integral cylinders, 80-inch Boxpok driving wheels, and Timken roller bearings on all engine axles. The Elesco feedwater heaters were moved to the top of the smokebox and an additional air pump installed, both of which were mounted on the pilot deck. Speed recorders were added. All 14 engines had already been equipped with Foam-meter, Batz engine truck, and force feed mechancial lubricators. The working boiler pressure was increased to 230 pounds. This rebuilding program was completed in May 1942 when Nos. 3751 and 3764 left Albuquerque. Nos. 3751, 3754, and 3755 were equipped with Worthington 6-SA feedwater heaters with hot water pump mounted on the pilot deck, similiar to the 3765 class. One air pump was moved underneath the cab on the fireman's side.

This rebuilding had a very beneficial effect on horsepower output. Steam pipes to and exhaust passages from the new cylinders had been increased to reduce the back pressure problems inherent in the original 3751 design. On four round-trip tests operated in December 1939 with dynamometer car 29 between La Junta and Los Angeles, handling trains 3-4, the California Limited, the maximum drawbar horsepower was 3600 in the 40-50 mph range with an indicated horsepower of over 3900. At 60 mph, No. 3751 developed only 2080 drawbar h.p., however rebuilt No. 3754 showed 3500. The 3751's output had decreased from 2680 at 55 mph, whereas No. 3754 dropped only from 3500 to 3350. The 3751 had not been tested at over 65 mph, at which speed the drawbar horsepower was down to 1560, while No. 3754 was still putting out 2610 at 75 mph. No longer were the 3751's considered lemons.

Concurrent with an order for its first two 5400 h.p. FT freight diesels, AT&SF ordered 10 additional 4-8-4's in July 1940. These 3776-class engines, Nos. 3776-3785 (construction Nos. 62465-62474), arrived during the summer of 1941. Baltimore & Ohio handled the engines from Eddystone to Chicago, but their size precluded them from being moved through the tunnel at Baltimore; indeed, it was necessary to remove them from the Baldwin plant on car floats. The 3776's were near-duplicates of the 3765 class. Multiple bearssheads were applied in lieu of the Laird type used on the 3765's and two air pumps were furnished, with the Worthington feedwater heater hot water pump moved to the left side of the boiler, directly back of the cylinder. Only one thermic syphon was provided, located in the combustion chamber. This reduced the firebox heating surface from 552 to 459 square feet. The most obvious change was the use of larger tenders, with eight-wheel trucks, carrying 24,500 gallons of water and 7170 gallons of oil. Nos. 3784-3785 were equipped with, Timken lightweight rods with roller bearing crank pins, and cost $182,833.64 each, compared with $168,760.03 for the other 8 engines.

Management felt that although the 3776's were acquired for passenger service, 4-8-4's could make a good showing on fast freight between Kansas City and Clovis. Although the dynamometer car was not furnished, No. 3776 left Emporia on August 2, 1941, on train 43 and ran through to Los Angeles, arriving on August 5. The elapsed running time of 45 hours, 37 minutes was at an average speed of 36.2 mph. The overall time of 53 hours was some 30 hours faster than the scheduled time of No. 43. Tonnage handled varied from 1273 tons on mountain territory to 2998 tons. Although it was recognized that the 4-8-4's did not develop sufficient tractive force at low speeds to be used on the heavy grades west of Winslow, plans were made to acquire 4-8-4's for fast freight service on more moderate grade territory.

With 35 4-8-4's available, the five daily transcontinental passenger trains were so equipped. The 3751's were assigned to Nos. 1-2 and 23-24 between Wellington and Los Angeles, and the 3765's/3776's were expected to handle Nos. 3-4, 7-8, and 19-20 between La Junta and Los Angeles. And the 3765's/3776's were expected to handle Nos. 3-4, 7-8 and 19-20 between LaJunta and Los Angeles. The Wellington-Los Angeles run encompassed 1542 miles on Nos. 23-24 and was the longest engine run in North America, if not the world. This run was established in September 1939, with 3751-class engines handling Nos. 23-24.