After you beat the Customs Fast Track mission in San Fierro, you'll be able to export and import vehicles by heading down to the freight ship at the docks there. If you've played either of the previous games in the GTA series, you should be familiar with this feature; you can earn a whole lot of cash by doing it, and will gain another notch in your 100% completion belt if you manage to successfully export all of the cars on the board near the boat.
The process of exporting a car is simple; when you get into one of the wanted cars, a message will pop up letting you know that the car is wanted down at the docks. If you drive it down there, you can either drive up the ramp to the deck of the ship, or use the crane to pick up the car and drop it into the red circle. When you do so, you'll gain a cash reward that varies based on the rarity/value of the car and the condition it's in; if you damage it a lot en route, you can lose thousands of dollars from your prize, so it may be worthwhile to hit up the Pay N Spray before stopping by.
There are three different boards, with ten cars each. You have to get all of the cars from the first board to unlock the second board, and then you'll be required to locate and deliver all of the second board's cars before you can move on to the third and final board.
|Board 1||Board 2||Board 3|
Some of these are fairly common, many are fairly rare. They won't all be found within the city limits of San Fierro, of course; some will only be located in the backcountry, some will only be in Las Venturas or Los Santos. You're going to have a heck of a time getting all of the cars for exporting, but if you do, you should manage to make around a million dollars for your efforts.
In addition to exporting cars, you can also import (buy) cars from the ship by walking up to the board and pressing the triangle button when prompted. You can find some obscure cars here, including those that you've already exported, but the cars are expensive, especially when you consider that you can jack any vehicle in the game world for free.
If you want to get to 100% game completion, you'll have to make ten successful deliveries in the Freight Train. Luckily, it's exceedingly simple to do so.
Before you can begin, you'll obviously have to be in a Freight Train, so head to a train station and wait for one to roll up. When it's stopped, jack it, then hit R3 to begin the mission. In it, you have to deliver five loads to five different stations within a certain window of time. X button accelerates, square button stops (or reverses, if need be). The trickiest part of the whole affair is managing the insanely long acceleration and stopping distances of the train itself; it can take you the better part of a minute to get up to top speed. Don't go too fast, though; at around 48 mph, you'll notice that your screen will start to shake, and if you keep accelerating beyond this point, you'll derail the train. You won't die from this, but you will have to start the delivery missions over.
Assuming you can learn how to stop the train within the delivery zones, though, you should be able to complete the missions without much of a trouble. If you're travelling around 40-45 mph, you'll need to slam on the brakes around 400 units of distance before the delivery zone to get yourself in position to stop. Once you get down to around 10 mph, ease up off the brakes and coast into the stopping position; the front car of the train has to stop within it before you can move on to the next delivery. The first level of deliveries has five stops, as does the second level; completing both will get you your mark towards 100% completion.