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 pounds 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.

The Chesapeake & Ohio's T-1 2-10-4 was so successful that it spawned a legacy of steam designs. The Advisory Mechanical Committee produced a scaled-down version for the Nickel Plate's first class S 2-8-4s of 1934; this basic "Van Sweringen Berkshire" design was replicated in locomotives erected for five other railroads, including the C&O itself, the last examples being outshopped by Lima in 1949 as the Nickel Plate's class S-3. Meanwhile the C&O's "Allegheny" 2-6-6-6, built by Lima beginning in 1941, is said to be a lengthening of the T-1 design, articulated so that it could negotiate the curves of the railroad's hilly territory. To crown the success of the C&O 2-10-4, the Pennsylvania Railroad, when faced with World War II locomotive design restrictions, based its 125 2-10-4s of 1942-44 on the same blueprint with only minor or cosmetic alterations. The Pennsylvania J1s, so unlike that idiosyncratic railroad's own experiments in late steam design, are often said to be its best modern steam power.