What do the pavement markings, such as an X and/or stop line show?

• May be marked on the pavement at the approach to some railway crossings.
  Explanation:

  "Pavement markings, such as an X and/or stop line, may be marked on the pavement at the approach to some railway crossings."
• May be marked on the pavement at the approach of a school bus.
• May be marked on the pavement at the approach of a heavy vehicle.
• May be marked on the pavement at the approach of a light vehicle.