There we were in the vaulted tunnel running,
You in your going-away coat speeding ahead
And me, me then like a fleet god gaining
Upon you before you turned to a reed
Or some new white flower japped with crimson
The woman, “you” is wearing her “going-away coat” that she would have worn at the end of her wedding, indicating that this is a newly married couple. 
 That she is “speeding ahead”, suggests that this is a part of the dream, a chase where the object is always out of reach.
The Underground