DCT Cabin John line
Along the line, just West of the B&O bridge.
EDIT 3/1/2019: NOTE - I spent a bit of time poking around and realized that Mr. Mumford's caption is a bit off. This photo was taken a bit southeast of the B&O's crossing of DC Transit's Glen Echo line. The location is actually where the DCT runs along Sherier Place and crosses Newark St. The houses have changed a bit (additions, subtractions, etc.) but the location is unmistakable. The proximity of the homes to the right of way are the real giveaway.
https://goo.gl/maps/B2AGTSh6vM42