This is all done in camera. I really thought it would look cool to have a light trail from a single boat traveling in the East River overlaid on the Manhattan skyline. Sounds easy... it was a real pain! It took multiple tries over a couple hour period to finally get it right. This is because it was hard to time the shutter speed and the timing of the shutter correctly. Some boats where moving faster than others and I had to time the shutter just right or another boat would pop into the shot just as the exposure would end. This would leave one random (and partial) light trail that didn't make any sense. If the shutter was too slow, the light trail would stop half way through the shot and I wouldn't capture the full light trail. I finally figured I needed ~30 secs to get a full trail across the part of the skyline I wanted, any longer and it would have been almost impossible because another boat would come zipping in leaving parts of a second light trail...
Gear
Body: Canon 6D Mark II
Lens: Canon 16 - 35mm f4L
Settings:
Shutter - 30 secs
Aperture - f22 (wasn't ideal but was needed to get a 30 secs exposure without blowing the highlights - should have had a 3 stop NDT filter...)
ISO - 100
Focal length - 20mm