Here is a Faq
Wrong Side of the Tracks
This is what I would consider to be the first somewhat difficult mission in
the game, and while it isn't particuarly hard by any means, it definitely has
the potential to be frustration. Drive Smoke's car to Unity Station for your
next set of instructions. After the scene hop onto the dirt bike with Smoke.
What you need to do is keep up with the train and take out the four guys on
top. Now you have to not only adjust your speed so you stay right beside
them, but you also have to adjust your distance so that you aren't too close
to the train. Being right beside the train the whole time will just lead Big
Smoke to hit the train itself with every shot, the best location is on the
right side of the train, and on the right side of the outer tracks. As you
keep going you'll see a car in the distance, don't be too close to the front
of the train when you approach it because it explodes when the train hits it.
Next danger is when you approach the tunnel, watch out for the oncoming train,
you should be safe if you're sticking with the right side of the tracks.
In the next area you need to ride up the dirt path on the side and stay close
to the edge of the cliff while Smoke fires upon the gangsters. Cross the
bridge to the other side and drive down the hill in front of the train so you
have the chance to get back on the right side. Now at this point the train
speeds up, and around the next bend is the last chance you have to finish them
off otherwise it disappears forever, so do what you can. When they're dead
just drive Big Smoke back to his house to get some well earned respect.