Location: This mission becomes available after the completion of the optional Vinewood Souvenirs- Al Di Napoli mission. You need to wait for the appearance of the new meeting location with Nigel. It is located in the central Los Santos, in the side alley in the Strawberry district (the above screenshot).
Listen to the conversation with Nigel that concerns the elimination of Al Di Napoli. Enter the old man's car (Declasse Premier) and drive towards the train station located in the eastern portion of the map. On your way, Trevor will start talking to Napoli and, if you want to make a quick buck, you can stop the car and let him out of the trunk. Know however, that this variant will make it unable for you to complete this mission in 100%.
A way better option is to ignore the man and complete your journey to the train station. Position the car exactly on the tracks (the above screenshot) and wait for a train to arrive (you can switch the camera to the cinematic mode). Note - if you want to complete this mission in 100%, you should jump out of the car, and away from the tracks, at the last possible moment. I recommend that you first master this variant, especially that if Trevor dies, you will only have to repeat the last dozen or so seconds of the mission. After you murder Di Napoli, contact Nigel.
Rewards for the completion of the mission:
- Cash (if you let Di Napoli out of the trunk)
Requirements for the completion of the mission in 100%:
- Skin of Your Teeth - You need to jump out of the car right before it gets hit by the train.
- Locomotivation - You need to have your part in murdering Al Di Napoli as a result of a collision of the car with the train.