It's a time exposure. The photographer set the camera on a tripod no doubt, with the shutter open for the whole time the plane was in view. Since it was night time, there wasn't enough light to ever sufficiently capture the image of the plane itself on the film, but each flash of the lights was captured, leaving the trail you see. It's hard to judge depth, so the climb may not be as steep as it looks, but it's a really interesting shot, nevertheless.
You've probably seen shots of cities at might, where the taillights and headlights of the cars look like streaks of light. Same principle here.
Thanks for posting the link.
Just noticed that PHL Approach posted pretty much the same info as I was typing this...
