SD24B. Illinois Central, the successor to the ICG, retired them in 1995. They later found work on shortlines such as the Chicago & Illinois Midland, the Wisconsin Southern (WSOR), the Iowa Interstate (IAIS), and the Indiana Harbor Belt. These units are slowly being retired, with the IAIS and WSOR having completely removed them from their rosters. The IHB has only 3 left, and the CIM (now the Illinois Midland) retains only 1 of their 5. Santa Fe SD26 From January 1973 through January 1978 the Atchison, Topeka and Santa Fe Railway rebuilt its fleet of eighty SD24 locomotives, naming the resulting type the SD26. The rebuild boosted the power output of the locomotives by upgrading the 16-567D3 engines with EMD 645 power assemblies. Other work improved the reliability by replacing the entire electrical systems, and replacing the multiple body-side vents with a central air filtration system. The large box fitted high on the body behind the cab for the air filters required relocating the "torpedo tube" air reservoirs further back on the long hood, and gave the SD26 a noticeable "hump-backed" appearance. One unit was wrecked, 44 were traded to EMD in 1985 and 35 were sold to Guilford Rail System in 1986. All have since been scrapped. Union Pacific SD24M The Union Pacific Railroad modified three SD24 locomotives experimentally to see if it would be worthwhile to upgrade their fleet of 30 such locomotives. Unit 410 received the most minor modification: upgraded air filters. UP 423, however, was substantially rebuilt to UP 3100 in August 1968 with a constant-speed 16-645 prime mover, a new alternator and new traction motors. With the constant-speed engine, speed control was via changing the level of excitation of the alternator; the traction motors were permanently wired in parallel. New, variable dynamic brakes were also fitted, as was a self load feature enabling testing under load without an external electrical load apparatus. This latter feature became standard on later locomotive models. The constant-speed prime mover, on the other hand, was not successful enough to duplicate. From 1975, the locomotive was used as a heavy switcher at UP's North Platte, Nebraska hump yards, a fate common to UP's other surviving SD24s. UP 414 did not receive as major a modification as #423, but it was refitted with a 16-645 engine, AR10 alternator and Dash Two electrical cabinet in May 1974, making it effectively an EMD SD40-2.