As this has spun off into its own thread, here are a couple more relevant photos. Amid all the trail frenzy I noticed one plane that wasn't leaving a persistent trail:
I then managed to fire up FR24 in time to get an identification, which explains it:
Air France
flight 11 from JFK to Paris was flying at 39,000 feet.
At the same time, and in the other direction, I watched a couple of planes at 36,000ft leaving trails from horizon to horizon.