A 49-year-old straphanger sneaking a smoke between subway cars was crushed to death after he somehow fell onto the tracks on the Upper East Side, according to cops and sources.The man was puffing away when he tumbled on the tracks and fell under the northbound 6 train near the 77th Street and Sixth Avenue station around 10 p.m.Christmas night on Wednesday, sources and police said. Medics found him dead underneath the train inside the tunnel between 68th and 77th streets, according to the NYPD. The Christmas tragedy delayed subway service. Cops did not release his identity pending family notification. Additional reporting by Amanda Woods...