Why do railways intentionally derail trains?

Railways intentionally derail trains using trap points to prevent a greater disaster. It is safer to derail a slow-moving train in a siding than to let it collide with a high-speed passenger train on the main line.

What is the difference between catch points and trap points?

Catch points are generally used on steep gradients to catch runaway vehicles rolling backward. Trap points are used at the exits of sidings to prevent unauthorized trains from entering the main line.

What is flank protection?

Flank protection is a signaling concept ensuring that trains on parallel or converging tracks cannot collide. Trap points provide physical flank protection by derailing any train that violates a stop signal.

The Lesser of Two Evils: Catch Points & Trap Points Explained

Catch Points and Trap Points act as the ultimate sacrifice in railway safety. Learn why intentionally derailing a train is sometimes necessary to protect the main line from collision.

December 9, 2025 11:59 am

What are Catch Points and Trap Points?

Catch Points and Trap Points are safety switches installed in the track specifically designed to intentionally derail a train or vehicle. While the idea of deliberately causing a derailment seems counterintuitive to safety, these devices operate on the principle of “the lesser of two evils.”

Their primary function is to prevent an unauthorized movement (such as a runaway train or a driver passing a red signal) from entering a busy main line and causing a catastrophic high-speed collision. By sacrificing the errant vehicle on the ballast, the system protects the lives of passengers on the main line.

The Concept of “Flank Protection”

In railway signaling, protecting the side of a train from incoming traffic is known as Flank Protection. Catch and trap points are physical guarantees of this protection. If a freight train tries to leave a siding without permission, the trap points will guide it off the rails into the ballast or sand, physically stopping it before it can “foul” (block) the path of an express passenger train.

Difference: Catch Points vs. Trap Points

Though often used interchangeably, their specific applications differ based on the track geometry and risk type.

Feature	Catch Points	Trap Points
Primary Risk	Runaway vehicles rolling backward.	Unauthorized exit from a siding.
Location	Steep gradients (hills).	Exits of yards, sidings, or depots.
Configuration	Usually spring-loaded (trailing).	Connected to the signaling system.
Goal	Stop a loose wagon from rolling down a hill.	Stop a train from entering the main line (SPAD).

Are They Still Used Today?

In modern railways, reliance on physical derailment devices is decreasing. Advanced Automatic Train Protection systems (like TPWS or ETCS) can automatically stop a train before it passes a danger signal, rendering the physical trap point unnecessary in many locations. However, they remain a critical last line of defense in freight yards and older rail networks.