Well, it seems that Delta is the official sponcer of the S train in NYC. All of Grand Central Station was filled with DL signs, posters, announcements, and advertisements. The S train (Times Square - Grand Central Shuttle) was especially flodded with Delta Ads. Every corner, beam, wall etc. had a DL ad on it.
Who is their advertising firm?
Did they change recently? Anywhere else you have seen them advertising aggresively?