This field guide highlights the Paleozoic geology of the Knoxville, Tennessee, area, framed in the context of the historic, halcyon days of Knox County’s marble industry and the railroads built to serve the area’s many limestone quarries and mills. The Three Rivers Rambler excursion train (the “Rambler”) is pulled by an 1890 “Consolidation” steam locomotive, which has been restored and is now operated by the Knoxville & Holston River Railroad Co., Inc. The Rambler route follows the north bank of the Tennessee River; passes through a sequence of Lower and Middle Ordovician carbonates, shales, and sandstones of the Knox and Chickamauga Groups; crosses the High Bridge at the confluence of the Holston and French Broad Rivers; and ends near Marbledale Quarry before returning to Knoxville. The two geologic groups are dominated by carbonates and lie in the syncline that contained most of the commercial marble that was quarried in the Knoxville area for the past 150 years. Exposures of the Holston Formation, a limestone commercially referred to as Holston [M]arble, were excavated to build the railroad ~125 years ago. It is possible to observe four of the seven formations making up the Knox and Chickamauga Groups along the route, but the outcrops are not accessible during typical railroad operations or by automobile. Arrangements were made for a field trip during the 2018 Geological Society of America Southeastern Section meeting, and this guide provides details for four selected exposures between Knoxville and the Forks of the River Marble District, with three optional stops. Only during the field trip, passengers will be able to disembark the train to examine carbonate and shale outcrops, structures, and discuss facies relationships of the foreland basin bryozoan reef deposits along the western flank of the Taconic (Sevier) foredeep. In addition to the local geology, this field guide describes the key role of railroads in the development of the Knox County marble industry, the history of what is today the Knoxville & Holston River Railroad, a corporate descendent of the 1887 Knoxville Belt Railroad Company, and the Tennessee Marble industry.