Union Pacific Nos. 998 and 999 were two DR-12-8-1500/2 (Centipede) type diesel locomotives built by the Baldwin Locomotive Works of Philadelphia, PA in December 1945. They were originally the two prototype units of this type of diesels built by Baldwin.
The three railroad companies that purchased Baldwin Centipedes include the Pennsylvania Railroad (PRR), the Seaboard Air Line Railroad (SAL), and the National Railways of Mexico (NdeM). The Centipedes were built between December 1945 and July 1948. Later on, the Union Pacific Railroad (UP) ordered the two prototype units from Baldwin where they were to be numbered as UP 998 and 999, but the UP eventually cancelled that order before they were ever delivered to them, so as a result of this, those units ended up eventually being scrapped.
The PRR and the SAL officially retired their Centipedes in the early 1960s and the NdeM officially retired their Centipedes in the late 1960s. Unfortunately, as of today, no Centipedes have been preserved, as all of them were eventually scrapped after retirement from service.