The MRL, Montana Rail Link, SD19-1 is a type of rebuilt, six-axle, 1,750hp diesel locomotive which is actually rebuilt from several EMD diesel locomotives (primarily the SD35 and SD9). Only two were built, numbered 651 and 652, and both units remain on MRL's roster.
During the early 1990's, the fairly-new Montana Rail Link US Class 2 railroad began to experiment with various different locomotives in their fleet. The majority of such units were from various different railroads (primarily from SP and BN heritage). Although the MRL had an impressive amount of various diesel locomotives to experiment with, only a handful were chosen. Two SD9 units (one from SP heritage, the other from EJE; Elgin, Joliet and Eastern Railway heritage) were selected and rebuilt as part of a short-lived program which was meant to serve as an economical form of avoiding the need to purchase completely new diesel locomotives which (for the MRL at the time) were deemed too expensive to purchase, maintain, and operate.
The program, however; didn't succeed due to financial issues, lack of resources, and the exceptions of leased units from various leasing companies (primarily CEFX or CITX; Cit Financial) being offered to the once-desperate railroad; thus, the discontinuation of the short-lived program.
Both units have remained in service at Fort Worth Texas in an intermodal facility since 2008-2009.
MRL #651 was originally built with components from a scrapped Union Pacific SD24, but proved to (obviously) become unreliable after awhile, and were eventually replaced with miscellaneous parts.
MRL #652 was primarily rebuilt with parts and components from a GP38-2 and an SD38-2.