REWARDS - Respect
Smoke wants you to take a ride with him, to a lead that might put you guys "deep in the game." Get to the red marker inside the parking lot from the Cesar Vialpando mission. You'll see the Vagos and the Mexicans having their meeting on a rooftop nearby, but they'll also take notice of you and the Vagos will jump onto the top of a train heading north.
You need to jump on the bike with Smoke and give chase. I suggest staying just to the right of the train tracks so Smoke can get a good shot at the Vagos on top of the train. He needs to shoot and kill all 4 on top. A bit down track will be a stalled car and the train will plow through it, so be sure to avoid the wreckage.
Just before the first tunnel will be a short train coming in your direction, so avoid it and continue pursuit. When Smoke suggests taking the high road, do so and follow the path. Keep your pursuit of the train and Smoke will eventually take down all 4 Vagos. Once the fourth is killed, Smoke suggests getting the hell out of there and wants you to take him back to his house. Once you get him back to his house, the mission is finished.