Another large class of Mikados was the 518 locomotives built for the Illinois Central between 1911 and 1923. There was nothing terribly exciting about these locomotives as built (Cylinders 27 x 30, drivers 63.5 inches, 185 psi boiler pressure, weight 285,570, starting tractive effort 54,158 lbs), what was interesting was what the IC rebuilding program did with this class.

The Illinois Central (the great rebuilder) received 518 2-8-2s of the same basic design between 1911 and 1923. (In addition, IC rebuilt 33 2-8- 0s into small 2-8-2s in the 1916 - 1924 period. Other 2-8-2s were acquired through mergers, and ten of the original group were transferred to the IC controlled Central of Georgia.)

The Illinois Central rebuilding program, which can be said to have started in the mid-1930s, was a major one, affecting almost every class of locomotive on the roster. As far as the Mikados were concerned, one can break down the rebuilding program into two parts: those locomotives, which were basically left as Mikados, and those, which were changed into something else. Also, many locomotives were left unchanged, but had received obvious improvements such as better lubrication systems and stokers. Finally, a warning: the tractive effort formula for two cylinder simple locomotives, a category into which all IC engines fell, used 85% of the rated boiler pressure as part of its computation. For reasons that are not entirely clear, perhaps an alteration in the valve gear which permitted greater admittance of steam at starting, or perhaps sheer happiness at the rebuilding program, the Illinois central decided to use a figure of 90% in its computations. Alert readers may wonder about the factor of adhesion, to which the only response would be a shared sense of wonderment.

In any case, the basic specifications of the 518 as built were cylinders 27 x 30, drivers 63.5 inches, working pressure 185 pounds per square inch. Starting tractive effort (at 85%) 54,158. Total weight of engine ranged from 282,700 to 298,000 lbs. The major rebuildings were as follows.

300 were rebuilt with the same specifications as above, with the boiler pressure increased to 225 psi and the tractive effort to 69,500 (90%, here and subsequently). The weight of this class was increased to 309,000 lbs. A further 100 were rebuilt with 28 x 30 cylinders, with a starting tractive effort of 75,000 pounds. Another 50 were given the same 28 x 30 cylinders and had their leading trucks removed to make them 0-8-2 switchers, with the same 75,000 lbs tractive effort.

In 1923, the Illinois Central had received 125 2-10-2 locomotives. Of this group, 56 had their boiler/firebox used for a conversion to 4-8-2 fast freight locomotives. Of the 56 "chassis" remaining 15 were converted to 2-10-0s, using 2-8-2 boilers and cabs. The specifications for these locomotives, mainly employed as hump pushers and transfer locomotives were as follows: cylinders 30 x 32, drivers 64.5 inches, boiler pressure 225 pounds per square inch, total weight 325,000 lbs, starting tractive effort 90,500 lbs.

41 more 2-8-2s kept that wheel arrangement, but had the 2-10-2 frame cut back. The result was cylinders 28 or 30 x 32 (13 engines), drivers 64.5, 225 psi boilers, starting tractive effort 76,900/90,000 lbs depending on cylinders fitted.