The Wilkes Street Tunnel is a small but satisfying piece of Alexandria's railroad past. Built for rail traffic and now folded into the walking and cycling network, it gives a quick, physical sense of the infrastructure that once threaded through the old port city.
It is not a destination on its own, but it is an excellent detail to include while exploring Old Town beyond King Street. Walk through it on the way to the waterfront or nearby historic streets, and you get a bit of industrial atmosphere without leaving the city core.