If the shape of Dalecarlia Tunnel looks odd, it may be because you are more familiar with the double track tunnels of the Old Main Line. Dalecarlia was single tracked, so is narrower.

According to this sign:

In the nineteenth century, boats along the Chesapeake and Ohio Canal transported coal from Cumberland, Maryland to Georgetown in the District of Columbia. By the early twentieth century, trains had replaced boats as a means of transportation and western Pennsylvania had replaced Cumberland as the source of coal. This spur route used by the Baltimore and Ohion, America's first railroad, later to become part of the CSX Corporation, followed the river downstream to Georgetown passing under MacArthur Boulevard by means of the Dalecarlia Tunnel. The Roman arch brick tunnel is 18 feet wide by 341 feet long. Architectural features of note are the four "step backs" or "duck ins" in the tunnel providing protection to track walkers caught in the tunnel at train time. The construction date, 1910, is prominent in the brick at both entrances to the tunnel.

The 1948 photograph of the south entrance to the Dalecarlia Tunnel is by E.L. Thompson.

Link to older pic: 1948

Disused but still tracked as seen in 1994. On this portal, the 1910 construction date surrounds the Dalecarlia name. In the north portal 19 and 10 are farther separated in the brickwork.

Late autumn colors make for a pretty scene in 2008 as an office building towers on the right.

Links to older pics: 1974, 1974

The Washington Aqueduct facility is connected to the Dalecarlia Reservoir.

Bridge 16 carries the trail over an aqueduct facility access road. The end portions of the bridge are of B&O-style contruction, there's even a painted on number 16, while the middle has been updated for trail purposes.

I believe the access road had previously been the Glen Echo trolley line.

Link to older pic: trolley in 1959

A warm "Welcome to DC" message is conspicuous by its absence.

Where the right of way meets a corner of the aqueduct facility a 1908-dated B&O boundary marker is still on duty. I've not seen this style of B&O marker used anywhere else in the region.

After passing the Dalecarlia water facilities, the rail-to-trail turns slightly east of south to cling to the side of a ridge into which the Potomac River has cut. The river and C&O canal babble unseen on the right until farther ahead.

At the intersection of Canal Road and Arizona Avenue (on the left), the B&O right-of-way begins its jump across to the Potomac River side of the C&O Canal.

The view from below reveals the crossing consists of two separate bridge segments. The direction of traffic flow along Canal Road's center lane varies based on time, warranting the unusual 5-headed traffic signals across the street.

The sun prefers to better light views in this direction. Where the bridges meet at the photographer's feet, a slight bend curves the tracks slightly more south.

Links to older picture: 1982

Many B&O bridges originating in the early 20th century received makeovers, but this one did not. The "whipple pin" cross members here echo those of the railroad's signature Bollman design.

Links: 1948, 1956

The C&O canal towpath is a great place from which to appreciate the strong elegance of the 1909-built bridge.

Links: 1966, 1977, canal mile marker, 2003 pics