A watershed design in steam locomotive technology was the Chesapeake & Ohio's T-1 class, represented here by No. 3009 in a photo of unknown provenance provided by Tom Rock of T.D.R. Productions. These forty 2-10-4s came off the Lima Locomotive Works erecting floor in 1930. At that time the C&O was controlled by the Van Sweringen brothers of Cleveland, who controlled several other railroads including the Erie. The Van Sweringens' Advisory Mechanical Committee apparently based the design of the C&O's T-1 on the Erie's S-1 2-8-4 of 1927, lengthening its dimensions. With 69-inch drivers — only an inch in diameter smaller than those of the Erie Berkshires — the C&O's T-1 was a higher-speed version of the "Texas" type than the previous, lower-drivered examples Lima had produced for the Texas & Pacific and the Chicago Great Western.

With its boiler stretching to 108 inches at its greatest diameter, and with a locomotive weight of 566,000 pounds, the C&O T-1 was the largest and most powerful two-cylinder locomotive in the world at the time of its introduction. These engines had 29x34-inch cylinders and carried 275 p.s.i. of boiler pressure, and exerted 93,350 pounds of tractive effort (108,625 pounds with booster cut in). An immense grate area of 108 square feet, together with an astounding 6635 square feet of evaporative heating surface and 3030 square feet of superheating surface, permitted these locomotives to steam freely at speed with heavy trains — the C&O being an important coal hauler.