Expert Track is appropriately named, definitely the hardest track in the game, so here's a guide to what I believe to be the optimum strategy. If you want to see these strats in action, look at my 15.54 run from the start to the excavator, and my 15.67 run from the excavator to the end. If all these steps go perfectly, a 15.3x should be possible. Also I have the steer sensitivity set to about 60%, which may take some getting used to.

1. Before starting the lap, go around the top of the second hairpin, as opposed to going under the arm of the excavator, and clip the silver barriers on the outside of the final hairpin. This will mean you start the lap at full speed.
2. Take the inner lane for the first corner, making sure not to crash into the tyres.
3. Turn the car as you go over the hill, so that when you land the car drifts around the corner tight enough to avoid hitting the tyre wall on the corner exit.
4. After avoiding the tyre wall, aim the car at the silver wall in the bottom right, before turning at the last second and hitting the brown wall at the spot just to the left of where it meets the silver wall. You should briefly tap the brakes just before hitting the wall to get the perfect speed for the next section.
5. Steer to avoid hitting the two tyre walls on the first hairpin, and aim for the excavator on the second hairpin.
6. Head to the left of the next two tyre walls without hitting them or the excavator, and go under the arm of the excavator, turning at the correct time to hit the silver barrier next to the 'Moonshine' billboard, which the car should bounce off of at full speed.
7. Another less consistent option here is to clip the tyre wall in the middle of the hairpin before turning, this will cause you to hit the tyre wall further down, which is usually about as fast, but has the potential to be even faster. However you have to clip the tyre wall just right, if you clip it too hard it will slow you down too much.
8. Steer to avoid hitting the next two tyre walls, aiming for the second silver barrier from the right.
9. Start turning at the right time to clip the silver barriers, this will get you around the final hairpin quicker than trying to take it as tight as possible, as the car doesn't slow down as much.
10. Stop turning once the car is pointing straight at the finish line to cross the finish line as quickly as possible.