"Jump onto the train" is an objective in the story mission Derailed in Grand Theft Auto V.
Walkthrough[edit]
Follow the road over until it starts to run parallel with the train tracks. Look along the right-hand side of the road and you will see a dirt ramp leading right up to the train:
Get on up the ramp, slack off the throttle a little so Trevor quickly reaches the end of the train. Move over to the right-hand side of the tracks and throttle back up. Watch the side carefully and you will see another ramp. Ride up it and jump from there onto the train.
If you miss, there are several more opportunities along the train tracks. If you are after Gold through, you need to get it on the first go.
