Playing as Franklin I think you get a call from Tonya again and she said someone is stuck on the train tracks. You get in your tow truck and look on your mini map and follow the blue highlighted part. Once you get to your destination you tow his truck from the front and drive around a little. The guy then gets in your truck and starts talking to you. You then follow the yellow part on your map and get to the destination and let the car off of your truck. He thanks you and you call Tonya and she thanks you and mission is over.