This photo is related to (intoduction and light trail) Posted by Dan Dare on the 25th Febuary.
I would guess that it is either dust, hair, cotton fiber or some sort of scratch, relating to your post and discussion. Depending on the type of camera you have and whether it appears in this one frame or on every frame, it may be also a film defect or an electronic (CCD chip of digital camera or equivalent) defect or aberration, random electronic noise, etc. Another guess could be a nocturnal bird, insect or bat that was flying during the shot. The best fix for the picture would be to crop it to eliminate the trail altogether then resubmit the photo.