Thomas Viaduct (Relay)

The primary viewing area is within the Avalon section of Patapsco Valley State Park, accessible from the contact station off South Street near Halethorpe. From the park road, you look across the Patapsco River valley at the full 612-foot curved bridge, with trains clearly visible as they cross the eight granite arches approximately 60 feet above the water. Trains cross at reduced speed due to the curve, giving extended viewing time per movement. The stone arches amplify wheel and coupler sounds, making approaching trains audible well before they appear on the bridge. CSX freight trains—intermodal double-stacks, manifest consists, autorack trains, and occasional unit trains—make up the bulk of traffic, with MARC Camden Line commuter sets adding variety on weekdays. The south bank along Levering Avenue in Elkridge offers an alternative angle looking up at the arches. Most railfans spend 1-2 hours and log several movements during weekday visits, especially mid-morning when both freight and commuter traffic overlap.

The viaduct spans a wooded gorge where the Patapsco River winds through mature deciduous forest typical of the Maryland Piedmont. The bridge deck sits roughly 60 feet above the river, and the surrounding terrain drops steeply from the approach embankments into the valley, creating natural elevated vantage points on both banks. On calm days the river reflects the arches, and morning mist frequently settles in the valley, adding atmosphere to early photography sessions. The Patapsco Valley is relatively quiet apart from distant highway noise from I-95, so the sound of trains on the granite spans carries clearly across the gorge. In autumn the warm brown Woodstock granite contrasts vividly with surrounding fall foliage; in winter, bare branches open up clear sightlines that summer leaf cover partially screens.

The Thomas Viaduct carries CSX freight traffic on the former B&O main line between Baltimore and Washington. Expect a steady flow of freight movements throughout the day: intermodal double-stacks, manifest trains, autorack consists serving Mid-Atlantic distribution hubs, and occasional unit trains. MARC's Camden Line adds commuter rail service on weekdays only, with trains running between Camden Station in Baltimore and Washington Union Station—the 39-mile route crosses the viaduct in both directions during morning and evening peaks. No Amtrak service uses this corridor; Amtrak's Northeast Regional and long-distance trains run via the separate Penn Line to the east. Weekend traffic consists entirely of CSX freight. Train lengths vary from short local cuts to long intermodal trains headed by GE or EMD power in pairs or triplets.

The Avalon Area of Patapsco Valley State Park on the north bank provides the most accessible vantage point. From near the contact station, a broadside angle captures the full 612-foot curved span with trains entering from the northeast. Morning sunlight illuminates the east-facing arches; afternoon light creates dramatic silhouettes. A trail descends toward the river, offering upward angles that emphasize the 60-foot height and the texture of the Woodstock granite masonry. From the south bank—accessible via Levering Avenue in Elkridge—late afternoon golden light bathes trains exiting the bridge against a backdrop of forested hills. Telephoto lenses (200mm+) compress the curve effectively, framing locomotives against the repeating arch geometry. The curved alignment means each train follows a visible arc across the full span, rewarding multiple passes with varied compositions. Tripods are welcome on park land.

Completed on July 4, 1835 for the Baltimore & Ohio Railroad, the Thomas Viaduct was the first multi-span stone masonry railroad bridge in the United States built on a curve. Designed by Benjamin Henry Latrobe Jr., the B&O's assistant engineer, its eight elliptical arches spanning 58 feet each were so ambitious that skeptics dubbed the project 'Latrobe's Folly,' predicting the 63,000-ton granite structure would collapse under its own weight. Named for Philip E. Thomas, the B&O's first president (1827-1836), the bridge instead proved revolutionary for American rail engineering. During the Civil War, Union troops heavily guarded both approaches because the B&O was the only railroad connecting Baltimore to Washington, D.C., making the viaduct a critical supply link for the Union. The bridge survived the devastating Patapsco Valley flood of 1868 and Hurricane Agnes in 1972. It was designated a National Historic Landmark on January 28, 1964, listed on the National Register of Historic Places in 1966, and recognized as a National Historic Civil Engineering Landmark by ASCE in 2010. It has carried trains continuously since opening day—191 years and counting.

Few active railroad bridges in North America combine continuous Class I freight service with pre-Civil War stone masonry construction. The Thomas Viaduct's curved alignment is rare among heritage structures—most historic viaducts run straight—and allows observers to see both the head and tail of a train simultaneously as it crosses. The acoustic properties of stone arches over water amplify every wheel tap and coupler clang, adding a distinct sensory dimension beyond visual observation. Its setting within Patapsco Valley State Park means legal, unobstructed access from public land with no fencing or trespassing concerns. An obelisk monument on the Relay side, erected in 1835 to commemorate the bridge's completion, adds a tangible connection to the earliest years of American railroading.