If a vehicle is stuck on a railway crossing and cannot be cleared, what warning measure does the PDR require when possible?

1. Send two people along the track in both directions at least 1,000 m, or one person toward the likely train direction if only one is available
2. Send one person 10 m behind the car only
3. Wait inside the vehicle and flash headlights
4. Place a warning triangle directly on the nearest rail and leave