B&O's tracks enter Union Dam Tunnel at left and emerge in shadow at upper left. Daylighting this tunnel is not possible because it is deep in the hill, and US 40, here under construction from bottom right, rides over it. The thin bright line at the river is Union Dam.

The original alignment continued ahead to the right then turned sharply left. The turn was so sharp even Union Dam Tunnel could not completely straighten it so it, too, curves to the left, though less abruptly.

Here's the view back from above the tunnel's portal.

Original OML alignments tended to hug the Patapsco's hillside undulations. One consequence is they often traced a route slightly inland of a tunnel's alignment.

Dave tells me these granite stringers, used instead of wooden ties to support rails in 1830, are southwest of Union Dam Tunnel. I have not been able to find them. They may have been covered by ballast added after the photo.

Link: MHT entry (PDF)

A few original granite stringers are still extant (below photo center) along the OML's original right-of-way bypassed by the tunnel. The existing tracks appear in the distance while the tunnel portal hides on the right.

The same stringers as seen from the opposite direction.

Union Dam, the tunnel's namesake, is in the distance.

A series of dams existed here to provide water power for the mill in Oella. The first dam was built in 1808. If it endured until 1868, it was probably ruined by that year's catastrophic flood, followed by a rebuild. If the replacement survived subsequent lesser floods, such as one in 1889, it might be the dam in this photo. Yet another dam was built soon after this photo. The masonry upstream at the far bank could be a remnant of a pre-1868 dam.

The 1868 flood undermined B&O's tracks here and precipitated a derailment. US 40 would span the river immediately upstream 24 years after this photo in 1936.

Link: source photo

B&O's original alignment curved within sight of Union Dam. The dam had been disused for at least 20 years when, in 1972, floods from Tropical Storm Agnes breached it and carved out the route around it seen here. Over subsequent years, the river washed away so much of the bank that stone-in-wire-mesh cages seen at the bottom were dropped along the shoreline to reduce erosion. The stone has obscured any surviving stringers in this spot.

Union Manufacturing Company's founder, William Patterson, for whom the Patterson Viaduct is named, would later become a director of B&O. A successor company, WJ Dickey, built the concrete dam seen here around 1914.

Link: 1833 trip from Baltimore

At low water, this does not look like a formidable obstacle for kayakers, but at other times it was: more than one person was trapped in the whirlpool and drowned.

In the distance is the bridge that carries US Route 40 over the Patapsco River between Baltimore and Howard Counties.

To eliminate the hazard and restore the river, work began in 2010 to remove the dam remnants. At photo time, landscaping finishing touches were underway. This photo was snapped close to the spot of the older one above. Removal has permitted the invasive snakehead fish to swim upstream.

Interpretive signage tells the story of the river, dam and mill. This can be accessed via a trail down from the Hollofield Area of Patapsco State Park, but steepness makes it more suitable for hikers than walkers. During the 19th century, westbound B&O trains passed right to left at this spot.

The 1936-built bridge for US 40 was reconstructed between 2010 and 2013.

The bridge crosses high above the Union Dam Tunnel. It required a fair amount of fill that has obscured much of the OML's original alignment underneath. The small stream on the far bank at right is named Miller Run.

This photo offers the reverse perspective, looking downstream from US 40 while the dam was intact. The railroad is hidden by the trees on the right.

Beyond the fill for US 40, a riverside path marks the original alignment. I saw no stringers here, but they likely remain buried under decades of leaves and mud.

The path emerges at the current railroad alignment. Compared to routing around the hill, the tunnel reduced the trip length by about 800 feet.

Under snow, the original alignment curves in from off photo left to meet the present day trackage.

Despite cutting off much of the curve, Union Dam Tunnel still forces a sharp bend in the rails at the tunnel's North (West) portal.