The fouling point is one of the rail warning signs designed for preventing vehicle collision. In response to the dispatching demands of trains or engineering vehicles, the fouling points are installed at the back end of the turnout crossing. The fouling points are used to regulate the drivers of trains or engineering vehicles to stop before the fouling point when entering the same turnout from different tracks to avoid collision with vehicles from adjacent tracks. Therefore, the locations of the fouling points need to consider the vehicle gauge and also the safety margin. Taking the TIAA MRT System as an example, the position of the fouling point is located between the two rails at the rear end of the turnout and is at least 3.8 m in horizontal distance from the two adjacent track centerlines plus the reserved vehicle sway. The fouling points of TIAA MRT main line are locked to the concrete track bed with 20x20 cm metal sheets; as for the fouling points of the depot areas are locked to the ballast bed with 20x20x40 cm concrete pillars. The surfaces are all coated with clear red paint to facilitate identification.