This is the Cabin John Bridge.
In 1884 the first recorded bicycle crossing on the parapet top was recorded in the local newspapers. The story quickly became quite famous and the etching on the right was included in a national publication later the same year.
This dare-devil feat was repeated many times over the years, up until at least the 1960's by several Cabin John youngsters. The addition of iron railings on top of the parapet finally brought the practice to an end.
I hope they were wearing helmets.