The Potomac Eagle runs on the South Branch Valley Railroad, a state-owned freight line that survived the broader 20th-century collapse of American branch railroading by becoming a public asset. The original line was built by the Baltimore and Ohio Railroad to serve the poultry and farming economy of the South Branch valley. When the B&O abandoned it in the 1970s, the West Virginia State Rail Authority took ownership in 1978 and established the South Branch Valley Railroad to keep freight service running between Green Spring and Petersburg, with stops at Romney and Moorefield. The freight operation runs weekdays and occasional weekends, which left the tracks open for a tourist excursion service. In 1989 the state began looking for an operator. The first Potomac Eagle excursions ran in 1991.

The signature stretch of the line is the passage through The Trough, the seven-mile gorge of the South Branch where the river runs in a single narrow channel between near-vertical rock walls. From the railroad's perspective the gorge is an ideal scenic asset - inaccessible by car, with no parallel road and few places to stop, offering passengers a view that can only be reached by water or rail. The Trough is also one of the most reliable bald eagle observation sites in the eastern United States. Multiple breeding pairs nest along the cliffs. The river holds fish. The cliffs provide updrafts. From the open observation cars passengers regularly see eagles flying alongside the train. The same gorge had two earlier roles in American history: as the site of the 1756 Battle of the Trough during the French and Indian War, and as one of the geological features that defined the South Branch's resistance to settlement well into the 19th century.

The Potomac Eagle's locomotive roster reads like a guide to mid-20th-century American passenger railroading. Current power includes an F7A unit numbered 722, originally built for the Bessemer and Lake Erie and recently repainted in Baltimore and Ohio livery. An FP9A numbered 1755 came from the Algoma Central in Canada. Two GP9s - 6604, an original B&O torpedo-boat passenger unit restored to its original colors, and 6240 in Chessie System paint from the Chesapeake and Ohio - haul most of the regular trips. A fifth GP9, number 8250, came from Canadian Pacific. Former engines have included an F3 numbered 8016 that has since been restored as Clinchfield 800, an ALCO FPA4 that was sold to the Grand Canyon Railway, and an FP7 sold to the Durbin and Greenbrier Valley Railroad. The Potomac Eagle has become a kind of working museum for early-generation diesels.

Excursions typically run from Romney north and south through The Trough to Moorefield, with occasional longer trips down to Petersburg. The shorter trip is about three hours round trip. Longer trips cover most of a day. Open observation cars, dining cars, and parlor cars rotate through the consists. The route passes by Civil War sites including Fort Mill Ridge and the South Branch valley battlefields, agricultural land that has been farmed continuously since the 1730s, and the broad bottomland that 1985 floodwaters covered when the South Branch crested at 22.6 feet at Franklin. The Potomac Eagle is one of the rare operations where the scenery, the wildlife, the equipment, and the route history all work together - and where a tourist trip ends up being a passable course in central Appalachian geography and human history.