It's possible this tunnel was rebuilt when concrete construction became popular on the WMRY (pre WW1). The original alignment was bypassed c.1938, when the current MD 550 was rerouted through the underpass by the Sabillasville P.O., and westward into Highfield/Cascade.
You are welcome to either contact or visit the WMRHS in Union Bridge for a more definite answer...
Erin,
Unless this bridge is a stone arch bridge that was encased in concrete, I would assume its newer than the 1870s as concrete most likely would not have been used at that time.
Was this underpass/bridge built in 1872 or later? Many thanks. Grew up next to this bridge in the old Sanatorium Power House Apartment.
Thank you both for responding. People always hit their horn driving through the underpass. I still do when I'm up that way. 😊