English: Reading Railroad steam locomotive 2100, undergoing refurbishment at the Midwest Railway Preservation Society in Cleveland, Ohio.

This locomotive was originally built with a 2-8-0 wheel arrangement (known as a "Consolidation"). This meant it had two unpowered wheels in a single truck, followed by six powered wheels (each with its own axle) and two coupled wheels. The locomotive featured a Wooten firebox with 262 square feet of heating surface, and 14" (356 mm) piston valves. Introduced in 1866, more than 23,000 of these locomotives were made.

The Reading ordered 2-8-0s between 1923 and 1925. The Reading model, known as the I-10 (or I10-sa) lengthened the firebox by a foot and added 91 square feet of heating surface. The engines used a 13" (331 mm) piston valve, and different running gear. created a home-built Northern.

In 1927, the American Locomotive Company (ALCO) for introduced a new locomotive, known as a "Northern" model, for use by the Northern Pacific Railway. These proved immensely popular, as the old 2-8-0 Consolidations had reached their horsepower limits.

The Reading's last 30 I-10s (originally numbered 2020 to 2049) were converted between 1945 and 1947 into Northerns. This meant changing the wheels to a 4-8-4 configuration (four leading wheels on two axles, eight powered and coupled driving wheels on four axles, and four trailing wheels on two axles). A new all-steel frame was used, with the cylinders cast as part of the frame. The smokebox was extended to 20 feet from 13.6 feet, and the boiler strengthened to 240 pounds per square inch of pressure from 220 pounds. Four thermic siphons were added to increase the efficiency of the boiler. These Northerns (because that is what they were) the Reading called its "T1" model, and they were numbered 2100 to 2129.

This T1 2100 is currently owned by the American Steam Railroad.