When in 1835 the B&O railroad built the Washington Branch from Relay to Washington, DC, they installed stone mile markers along the route. Engraved into each stone is the distance from Baltimore and Washington. The values always total 40, since at the time, that was the distance between the B&O's primary passenger stations in those cities.

It is not known if any markers between Baltimore and Relay (the first 8 miles) are still extant, or if they ever existed. Research is in progress. The map grid locations given below are the locations or expected locations of the markers.

Some of these original markers between Relay and Washington survive to the present. All the known ones are located on the southeast side of the tracks, except at Relay which has markers on both sides. A steel "S" post can be found at some of the mile locations. I've been told the S posts designate the start/end of a measured mile for the purposes of locomotive speed tests.

The table below shows the status of the markers. A ? indicates a location I have not yet checked.