There are several ways of dealing with the Train fight. The Trains speed up and slow down based on your proximity to them, so you can manipulate this to get within spraying distance. They also always slow down when they're shooting electricity, making that an opportune moment to attack.
The train will stay at approximately 250 speed (grinding speed) until it begins to perform an attack. As an attack is ongoing, the train will slow down slightly, allowing the player to catch up and spray it. If the player is on a rail that is being attacked, a trick jump will cost the player too much speed, and they will not be able to catch up to the train on that attack cycle. Keep this in mind if you find yourself having to improvise a route during the train fight.
If the player stops far enough behind the train, the train will stop moving. The player can hop on any rail, wait a second, boost, and catch up to the train as it starts to acceleration. If done too quickly, the player will hit the back of the train and be knocked down.
Enter the fight with at least 30 cans. By this point in the run, you should be playing as a 35-can character.
Grind the far right rail and pick up the 5 cans. Spray the train as soon as you can until it's dead.
Grab the 5 cans, then jump to the middle-right rail while spraying the 2nd train - land just in front of the electricity and boost. Spray the train as soon as you can till it's dead.
Grab the next 5 cans on this rail. As soon as you grab the 5th can, boost and jump hard left so you can take the corner as tight as possible. Spray the far left train while you're doing this. Once you're back on the rail, continue spraying it till it's dead.
It helps to run along the left side of the far left rail, as you move at a higher speed than boost-grinding - this will help you catch up to the train.
Once the 3rd one is dead, short hop to the middle-left rail and chase it down (don't boost). The short hop will make the train slow down, allowing you to catch up once you're back up to speed.
This is a slower but easier strat that takes care of the trains from right to left. This can be done with a 30-can character.
Grind the far right rail and pick up the 5 cans. Spray the train as soon as you can until it's dead.
Grab the 5 cans, then jump to the middle-right rail - land just in front of the electricity and boost. Spray the train as soon as you can till it's dead.
Grab the next 5 cans on this rail. As soon as you grab the 5th can, jump to the middle-left rail and just keep doing tricks till you catch up. Spray it till it's dead.
Wait till the train releases its bomb. Jump over the bomb and land on the far-left rail. Spray it till it's dead.