A carriage of Vagos is leaving Los Santos by rail with a shipment on board, and Big Smoke's answer is a single dirt bike with Carl driving and himself on the pillion. The mission is a chase along the track out of Unity Station with Smoke shooting at the front carriage and Carl doing everything else.
It is the most notorious mission in San Andreas, and the reason is entirely mechanical: Smoke's accuracy depends on where Carl puts the bike, and nothing on screen says so. Get the position right and it is over in a minute.