OFHX 1258 is a Whitcomb RS-4-TC. This unit (like all RS-4-TCs) was built for the US Army, weighs in at about 48 tons, and produces about 500 hp. 1258 specifically was constructed in October of 1953. The unit is now owned by Fluor-Daniel-Fernald, and previously was working the cleanup of an old uranium enrichment plant at Fernald, OH. It's now headed to near Moab on the Potash Branch to assist with cleanup of uranium tailings along the Colorado River.
From what I've read, the unit is considered contaminated, and will be stored in a fenced yard near the end of the Moab Branch when not in use. It will also be joined shortly by a twin - OFHX 1250. 1258 was in Grand Junction on Thursday, 19-Apr-2007, where Matt and Wayne Darling managed to get these photos of the unit. Thanks to both of you.