The original Transcontinental Railroad was completed around the north shore of Great Salt Lake, Utah, in May 1869 at Promontory, as part of the Salt Lake Division of the Central Pacific Railroad, which became the Southern Pacific. It was the primary main line for the Overland Route until around 1904 when Southern Pacific's Lucin Cutoff was completed across Great Salt Lake, which offered a shorter distance and easier grades through the Promontory Mountains. The original line, between Umbria Jct. (Lucin) and Corinne, Utah was downgraded to "secondary" status, and was used for passenger traffic only.
Technically, the part of the line east of Promontory was part of the UP but transferred to the Central Pacific/Southern Pacific around 1870 when the "transfer point" was moved from Promontory to Brigham City, Utah. (Brigham City was a better location for servicing trains.) With the UP's acquisition of SP it reverted back to UP after almost 130 years.
The tracks were abandoned in 1942 as the rails were needed to support the war effort during World War II; specifically, most of the track and hardware were relocated to military bases on the Pacific Coast.
For about 100 miles or so west of Ogden, and particularly in the completely deserted area west of Promontory, the barely-used UP grade is clearly visible parallel to the maintained CP grade.
The grade is maintained now by the Bureau of Land Management as a National Backcountry Byway, and is therefore in excellent condition, also due to the near desert-like conditions in the area. Some portions of the line do cross private property.
Interestingly enough, there were actually two grades built, both parallel and adjacent to each other. Both the Union Pacific and Central Pacific were under contract by the U.S. Government to build, and each were paid by the mile. It wasn't until after the construction crews passed each other (working in opposite directions) that the meeting point was established. The Central Pacific grade is now the famous one, and is the one preserved by the BLM. The Union Pacific grade was abandoned in 1870, less than a year after the Golden Spike was driven, qualifying it as one of the oldest abandoned grades that it still clearly visible.
Towns and locations along this former ROW are: Corrine to Quarry, Balfour, Conner, Lampo (Blue Creek), Surbon, Promontory, Rozel, Lake, Kosmo, Monument, Nella, Kelton, Peplin, Ombrey, Matlin, Terrace, Watercress, Bovine and to Umbria Junction on the SP/UP. Between Promontory and Rozel, a record 10 miles of track was laid on April 28, 1868. Track has been reinstalled on some of the ROW around the Promontory National Historic Site.
The Map of this route depicts both the CP and UP abandoned lines, with the CP being the northern of the two.