During the first decade of the 20th century, Lemhi County, in the remote Salmon River country of central Idaho, was seeing both increased agricultural development and substantial, renewed mining activity.

The desire for a railroad was perhaps most strongly felt by those promoting the county's mining developments, although a variety of existing railway companies also studied the prospects of the area.

The NP gave Lemhi County particular attention, envisioning a new transcontinental route that would veer southward from their existing lines in Montana, cross into Idaho via Bannock Pass, and follow the rugged Salmon River canyon westward across the state.

These dreams began manifesting in 1907 when a group of Pennsylvania businessmen led by W. A. McCutcheon incorporated the Gilmore and Pittsburgh Railroad.

In the valley, branch lines would run upstream to the promising mining camp of Gilmore, and downstream to the county seat of Salmon.

Construction was pursued in earnest throughout the following year, working west from Armstead, and the tracklayers reached Salmon on April 25, 1910.

McCutcheon had promised that the line would be extended northward from Armstead to Dillon, Montana that year, the first step in completing a connection to the NP's trackage at nearby Twin Bridges.

Descending Bannock Pass, the railroad followed a narrow canyon into Idaho's Lemhi Valley and the small agricultural community of Leadore, some 55 miles (89 km) from Armstead.

The main line continued northwest from Leadore for another 45 miles (72 km), following the broad valley and the Lemhi River downstream through fertile farming and ranching land to the county seat of Salmon.

Gilmore in 1910 was the center of a prosperous silver and lead mining district, and was thus a major source of early traffic for the railroad.

The railroad's equipment roster was significantly reduced over the years, as well, because of a lack of traffic and a need to conserve expenses.

The railroad's lack of economic viability was realized as early as 1913, when the G&P's substantial construction indebtedness (nearly $6,000,000) was written off by the Northern Pacific in exchange for full ownership of the line.

The railroad's prospects worsened in the 1920s, as the Gilmore mines declined and eventually closed, and in the 1930s as improved local roadways made auto traffic to and within the region easier.

The moribund railroad sat unused until the Interstate Commerce Commission approved the line's abandonment in 1940, and the track was removed later that year.