 Railroads started rebuilding their RS-3 fleets early on because of the unreliability of the ALCO 244 prime movers.The standard practice among railroads which rebuilt diesel locomotives in a manner which differed from its original configuration was to add to its original model designation an "m" for modified, an "r" for rebuilt, or a "u" for upgraded, although some railroads created a new designation, as when the Green Bay & Western reclassified its upgraded RS-3's as RS-20's.Perhaps one of the earliest examples of the RS3m is the former Missouri-Kansas-Texas Railroad's RS3m fleet.