By Lloyd E. Stagner and TRAINS Magazine

New, modern high speed passenger locomotives were authorized by the Board of Directors in November 1936. Eleven 3765-class 4-8-4's would provide the needed speed and capacity for the mountain territory west of La Junta, and six 3460-class 4-6-4's would work between Chicago and La Junta. Ten 2-10-4 freight engines were also authorized.

Again, Mechanical Engineer Lanning and his staff collaborated with Baldwin in the design of the 3765's, which were delivered between April and June 1938. For maximum speeds of 100 mph, 80-inch Baldwin disc-type driving wheels were specified. Cylinder dimensions were changed to 28 x 32 to eliminate the back pressure problems inherent in the 3751 design. Wagner cylinder bypass valves provided for effective lubrication while the engines were drifting down long grades. Three mechanical lubicators provided oil to all driving-box and engine-truck pedestal faces, the two main guides (which were of the Santa Fe standard Laird two-bar design), cylinders and piston valves, and the feedwater heater hot water pump, which was mounted on the right side of the pilot deck, with the air pump on the other side.

The boiler design was similar to the 3751's, with the same 108-square-foot grate area, 64-inch combustion chamber, and 20-foot, 11-inch flue length. The most notable changes in the boiler were the application of three Thermic syphons instead of one and an increase in working pressure to 300 pounds with the use of nickel steel in the boiler construction. The feedwater heater was of the most advanced design available, the Worthington 6-SA, which afforded higher capacity and higher temperatures than the Elesco model used on the 3751 class. The Santa Fe patent Layden exhaust nozzle, which separated the exhaust blast from each cylinder, was used.

A one-piece cast engine bed with integral cylinders was furnished by General Steel Castings Corporation. Engine trucks were of the Batz design and trailer trucks were of the GSC Delta type. Walschaerts valve gear, controlled by a Baldwin Type C power reverse, with a maximum cutoff of 60 per cent was used. Tandem main rods connected the second and third drivers. Timken roller bearings were used on all engine axles. Other special equipment included American multiple throttle, Weston speed recorder, Foam-meter with two automatic blow off cocks located above the mud ring at the back corners of the firebox, throat sheet blow off cock, Muchnic bronze cylinder packing, and drum-type fuel oil heater. Total engine weight was nearly 250 tons at 499,600 pounds, of which 286,980 were carried on the driving wheels.

Tender capacity was 20,000 gallons of water and 7107 gallons of fuel oil, carried on Pullman six-wheel trucks equipped with Timken roller bearings. Total weight of the fully loaded tender was 396,246 pounds.

Starting tractive force was calculated at 66,000 pounds; Test Report 87312, "Performance of Locomotive 3766," however, had the following observation: "Both the indicated and drawbar horsepower exceed that corresponding to the theoretical tractive effort and drawbar pull throughout the speed range showed an average m.e.p. of 240 pounds, which is 80 per cent of boiler pressure, giving a calculated tractive force of 75,200 pounds based on 61.5 per cent cutoff with a factor of adhesion of 3.8." AT&SF did not follow the recommended 77 per cent of steam pressure in the formula of the Mechanical Division of the Association of American Railroads, which would have equated to 72,400 pounds tractive force. The 3.8 adhesion factor resulted in the 3765's slipping starts with the heavy freights they pulled in later years.

Nos. 3766-3775, construction Nos. 62120-62129, were delivered in April and May 1938. The first of the class, No. 3765 (construction No. 62119) was the last to arrive on Santa Fe property, in June. Original plans had been to place a streamlined shroud on this engine similar to 4-6-4 No. 3460. After much discussion with Baldwin, the plan was dropped because of the additional weight involved. No. 3765 did receive nickel steel inside firebox sheets, aluminum hand rails, and a Sunbeam instead of Pyle National headlight generator in preparation for its shroud.

No. 3766 arrived at Topeka April 21, 1938, for testing and setup. Road tests commenced with a dynamometer car on May 12, running to La Junta in freight service at speeds restricted first to 20 mph, later to 40 mph. One round trip in freight work was operated to Albuquerque, westbound on No. 31, and returning on a banana special of 30 cars. A round trip on Nos. 23-24 between La Junta and Albuquerque was 3766's first passenger assignment, followed by a round trip to Winslow and the handling of No. 3 through to Los Angeles on June 11-13. A round trip from Los Angeles to Winslow, eastbound on the Chief and returning on the Scout, followed, then two runs up Cajon Pass. On June 18, the 3766 handled the Navajo, consisting of 10 cars, 841 tons, to Summit, unassisted on 2.2 per cent grade and made the schedule time of 1 hour at a speed of 22 to 30 mph. The report connnented that this "represents very good performance" and "no slipping was experienced." After a trip to Albuquerque on No. 8, No. 3766 entered the through La Junta-Los Angeles passenger engine pool which had been established with 3751 class on December 7, 1937. On this 1236-mile run, nine engine crews handled the 3766, and with the high-capacity tender, fuel oil was taken only at Albuquerque and Hackberry, Ariz., or Needles, Calif. In 1938, eight daily passenger trains were scheduled, and the 25 4-8-4's protected most of them, plus Nos. 23-24 between Los Angeles and Winslow. The 3700-class engines continued to protect Nos. 23-24 east of Winslow and Nos. 1-2 between Clovis and Los Angeles.