From Dave Brigham:
I've passed this narrow, old bridge in Belmont, Mass., countless times in the 20+ years I've lived in and around Boston. It's one of those places that doesn't look like much, but surely has some stories to tell.
I wish I could relate some of those stories to you, but I don't know what they are. But I can tell you a little bit about this bridge.
I trolled the Internet, as I often do seeking information for these posts, and learned that the Clark Street bridge at one time was wide enough to accommodate auto traffic traveling from Route 60 (Pleasant St.), across the train tracks and into the quiet neighborhood just south of Belmont Center. Or perhaps from that neighborhood out to Pleasant Street, as it seems it was a one-way street.
Eventually, both ends of the street were blocked, leaving room for folks to walk or bike across. At some point, the crossing was modified to just a foot bridge.
So, yeah, that's all I know.