Blinking light moving very fast in an airplane contrail.I'm not sure if the date is exactly the right. A few days in both directions at most. I was sitting out on my balcony taking some sun as it was a clear warm cloudless day. As I took a break from it I watched an aircraft fly over me at cruising altitude leaving a contrail. There is a flight corridor right overhead (west -> east) for most likely intercontinental flights. About five to ten minutes after the aircraft went out of sight (balcony's ceiling). I noticed a bright white light moving thru the sky. It was a constant blinking light in about one second interval, moving in my point of view in the exact same place and direction as the contrail left by the recent aircraft. But the speed was 6 - 8 times faster (or even more) than the normal aircrafts that slowly move across the sky. There were no change of speed what so ever. I didn't see any object during the "no light" period. Maybe it was behind the contrail. But the blinking light was so strong it seemed to be below the contrail. This made me think this thing was actually moving IN the contrail. The contrail wasn't affected in any way as it was slowly disappearing. The size of the light was about twice of a big airliner flying at cruising altitude. I'd say the light moved over the part of the sky I can see (about 40% of total) in about 15 seconds.The huge speed, bright blinking light, moving exactly where the contrail was, made no sense to me and still I'm wondering what it was. ((NUFORC Note: Date is approximate. PD))