Question: Which American railroad, located in Southwestern Montana and Idaho, was backed by the Northern Pacific Railway?

Context paragraphs:
Northern Securities Company: The Northern Securities Company was a short-lived American railroad trust formed in 1901 by E. H. Harriman, James J. Hill, J.P. Morgan and their associates.  The company controlled the Northern Pacific Railway; Great Northern Railway; Chicago, Burlington and Quincy Railroad; and other associated lines.  It was capitalized at $400 million, and Hill served as president.

Billings Depot: The Northern Pacific Billings Depot is a historic train depot in the Historic District of downtown Billings, Montana.  The depot was constructed to serve as a passenger station for the Northern Pacific Railway, Great Northern and Chicago, Burlington and Quincy.  All three railroad merged to form the Burlington Northern Railroad in 1970, along with the Spokane, Portland and Seattle Railway.  In 1971 Amtrak took over passenger service throughout the country.

Montana Railroad: The Montana Railroad was an American railroad built and operated between the towns of Lombard and Lewistown, Montana, a distance of approximately 157 miles.  The railroad connected with the national railway network via a connection with the Northern Pacific Railway at Lombard.  The Montana Railroad line was constructed between 1895 and 1903, and operated independently until 1908, when it was acquired by the Chicago, Milwaukee, St. Paul and Pacific Railroad ("the Milwaukee Road").  The railroad was colloquially known as "the Jawbone", because of the contrast between the promising statements of the line's promoters and the company's perennially-weak financial position.

The Murray Hotel: The Murray Hotel, originally named the Elite Hotel, is an historic hotel in Livingston, Montana, United States.  The original two story hotel was built at the corner of Park and Second St. in 1904 by Josephine Kline to accommodate passengers from the Northern Pacific Railway.  The Elite was one of thirteen hotels built in downtown Livingston between 1884 and 1914 to service railroad travelers.  Its construction was financed by the family of a future U.S. Senator from Montana, James E. Murray.  The hotel's location opposite the Northern Pacific Railway's Livingston Depot made it a prime destination for railroad travelers.  The hotel is located within the Livingston Commercial District, a registered National Historic District.

Oregon, Washington and Idaho Railroad: The Oregon, Washington and Idaho Railroad built a 72.03 mi rail line along the right (north) bank of the Snake River between Riparia, Washington and Lewiston, Idaho.  The company was incorporated in Oregon on August 8, 1903, and began operating its completed line on July 7, 1908 as an operating subsidiary of the Union Pacific Railroad (UP), which also controlled the Oregon Railroad and Navigation Company (OR&N) through Riparia.  A portion of the preliminary work had been done by the Snake River Valley Railroad, which built a completed line along the Snake River below Riparia.  Beginning on December 3, 1909, the Camas Prairie Railroad, a joint subsidiary of the OR&N and Northern Pacific Railway (NP), began operating the Oregon, Washington and Idaho Railroad (OW&I) as part of a line between Riparia and Grangeville, Idaho, including segments owned by the NP and subdidiary Clearwater Short Line Railway.  On December 23, 1910, the property of the OW&I was conveyed to new UP subsidiary Oregon–Washington Railroad and Navigation Company, successor to the OR&N, but the Camas Prairie Railroad continued to operate it as agent.

Northern Pacific Depot: Northern Pacific Depot, or Northern Pacific Railway Depot, or Northern Pacific Passenger Depot, or Northern Pacific Railroad Depot, or Northern Pacific Railway Passenger Depot may refer to the following stations in the United States:

Wadena Depot: The Wadena Depot is a historic former railway station in Wadena, Minnesota, United States, operated by the Northern Pacific Railway from 1915 to 1971.  The station was listed on the National Register of Historic Places in 1989 under the name Northern Pacific Passenger Depot for having local significance in the theme of transportation.  It was nominated for representing the impact of the Northern Pacific Railway on the establishment and development of Wadena.

Gilmore and Pittsburgh Railroad: The Gilmore and Pittsburgh Railroad (G&P), now defunct, was an American railroad located in southwestern Montana and east-central Idaho.  Constructed in 1909 and 1910 between the towns of Armstead, Montana, and Salmon, Idaho, the G&P served mining and agricultural areas in Lemhi County, Idaho, and Beaverhead County, Montana.  The line was financially backed by the Northern Pacific Railway, and later became its subsidiary.  Never financially successful, the G&P ceased operations in 1939, and the railroad was dismantled the following year.

Lombard, Montana: Lombard is a ghost town in southeastern Broadwater County, Montana, United States.  The town was located on the east bank of the Missouri River, just north of the mouth of Sixteen Mile Creek.  Lombard was established in 1895 as the western terminus of the Montana Railroad, and the location of its interchange with the Northern Pacific Railway.  In 1908, the Montana Railroad was incorporated into the new transcontinental main line of the Chicago, Milwaukee, St. Paul and Pacific Railroad ("the Milwaukee Road").  This lessened Lombard's importance as a railroad operational base, but the town survived as an interchange point between the Milwaukee and the Northern Pacific.

Northern Pacific Railway: The Northern Pacific Railway (reporting mark NP) was a transcontinental railroad that operated across the northern tier of the western United States, from Minnesota to the Pacific Northwest.  It was approved by Congress in 1864 and given nearly 40000000 acre of land grants, which it used to raise money in Europe for construction.

Supporting paragraphs: Gilmore and Pittsburgh Railroad Northern Pacific Railway
Answer: Gilmore and Pittsburgh Railroad