To begin this mission, you will need to have gotten Tonya as a contact for your phone. Once you have, call her up and she'll have you doing another job. Go to the Towtruck as usual, but this time it doesn't involve an abandoned vehicle. It seems a car has stalled on some train tracks, so head to its location.
You don't have much time to get the car hooked up, as the train will hit it in a few seconds. Since you're likely to be coming in from behind, you should be able to line up the Towtruck with the car where you only need to back straight up to hook it. Once you have, get it clear of the tracks, then stop to let the driver inside the Towtruck. Drop the car off at the auto repair shop to finish the mission.