Today's shoot was in a beautiful Montana train yard.  Recording clean dialog here was a challenge.  One of the representatives from the rail company was kind enough to shut down the closest engine which reduced some of the noise.  I discovered that most of the noise came from the SIDES of the closest engines.  I worked with the team and we decided to go with a good looking shot from the front of the train, which put the loudest noise BEHIND our talent and their body provided a barrier between the noise and the microphone on their chest.  The boom still sounded the best, but the lav on their chest gave us the best dialog to noise ratio.