The Weather in the Streets by Rosamond Lehmann

Taking up where Invitation to the Waltz left off, The Weather in the Streets shows us Olivia Curtis ten years older, a failed marriage behind her, thinner, sadder, and apparently not much wiser. A chance encounter on a train with a man who enchanted her as a teenager leads to a forbidden love affair and a new world of secret meetings, brief phone calls and snatched liaisons in anonymous hotel rooms.