That is indeed the Dalecarlia rail tunnel down by the reservoir on the DC line. I lived in Bethesda for quite a few years and used to ride the Crescent trail on my Raleigh Sports and Columbia ballooner. The trail was usually pretty busy on weekends, but weeknights were more manageable. The tree roots used to make bounce pads under the pavement. But it was a lot of fun hitting the trail 5x per week. That was quite a few years ago now though...