Between 1937 and 1943, the Paducah shops took the boilers of 56 class 2900 2-10-2 locomotives and built new 4-8-2 locomotives of the class 2500 with them. The dimensions of the boilers were not changed, with a firebox heating surface that included 23.5 square feet in arch tubes, 108.5 square feet in thermic syphons and 386 square feet of the actual firebox walls. At the same time the frames, drivers and cylinders were used for the class 2100 2-8-2 and class 3610 2-10-0.

The first locomotive was built with a BK stoker, but all others got Duplex stokers. Only No. 2540 used Timken roller bearings on its axles. The 17 first locomotives had the same tenders as the original class 2900, with 16 tons of coal and 12,000 gallons of water. 20 Others got bigger ones with 20 tons of coal and 15,000 gallons of water. The remaining ones got tenders with 24 tons of coal and only 11,000 gallons of water. But these locomotives additionally had auxiliary water cars. A big change was the boiler pressure that had been increased from 190 to 225 psi. Later the boiler pressure of all was increased to 240 psi, in parallel with the remaining class 2900 locomotives.