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Catch points andtrap points are types ofpoints which act asrailwaysafety devices. Both work by guidingrailway carriages and trucks from a dangerous route onto a separate, safer track. Catch points are used toderail vehicles which are out of control (known asrunaways) on steep slopes. Trap points are used to protect main railway lines from unauthorised vehicles, moving onto them fromsidings orbranch lines. Either of these track arrangements may lead the vehicles intoa sand drag or safety siding, track arrangements which are used to safely stop them after they have left the main tracks.
Aderail is another device used for the same purposes as catch and trap points.
Catch points are used where track follows a risinggradient. They are used to derail (or "catch") any unauthorised vehicles travelling down the gradient.[1] This may simply be a vehicle that has accidentally been allowed to run away down the slope, or could be a wagon that hasdecoupled from its train.[2] In either case, the runaway vehicle could collide with a train farther down the slope.
Catch points may consist of a fullturnout or a single switch blade. In some cases, on atrack that is only traversed by uphill traffic,trailing point blades are held in a position to derail any vehicle travelling downhill. However, any traffic travelling in the correct (uphill) direction can pass over theturnout safely, pushing the switch blades into the appropriate position. Once the wheels have passed, the catch points are forced back into the derailing position bysprings.[2] In these cases, a lever may be provided to temporarily override the catch points and allow safe passage down the gradient in certain controlled circumstances.
The use of catch points became widespread in theUnited Kingdom after theAbergele rail disaster (1868), where runaway wagons containingparaffin oil (kerosene) collided with anexpress train. Catch points continued to be used in the UK until the mid-20th century. At this time,continuous automatic brakes, which automatically stop any vehicles separated from their train, were widely adopted, making catch points largely obsolete.[citation needed]
Trap points are found at the exit from a siding or where a secondary track joins a main line. Afacing turnout is used to prevent any unauthorised movement that may otherwise obstruct the main line.[1] The trap points also prevent any damage that may be done by a vehicle passing over points not set for traffic joining the main line.[2] In theUnited Kingdom, the use of trap points at siding exits is required by government legislation.[2]
An unauthorised movement may be due to a runaway wagon, or may be a trainpassing a signal at danger. When asignal controlling passage onto a main line is set to "danger", the trap points are set to derail any vehicle passing that signal.Interlocking is used to make sure that the signal cannot be set to allow passage onto the main line until the trap points have been aligned to ensure this movement can take place.
Trap points should preferably be positioned to ensure that any unauthorised vehicle is stopped a safe distance from the main line. However, due to space limitations, it is not always possible to guarantee this.
If the lines aretrack circuited and a wagon or train using the catchpoint could foul an adjacent line, then atrack circuit interrupter will be fitted to one of the run-off rails. When a train runs off it will break the track circuit and set main line signals to 'danger'.
There are several different ways of constructing trap points:[2]
The type of trap points to be used depends on factors such as thegradient of the siding, and whetherlocomotives enter the siding.[2]
In some cases, catch points and trap points direct vehicles into asand drag orsafety siding, also sometimes called anarrestor bed. This may be a siding simply leading to a mound ofsand,gravel or other granular material, or a siding where the rails are within sand-filled troughs.[1] This method of stopping a vehicle travelling at speed is preferred over a buffer stop as there is lessshock to the vehicle involved.[2]
As catch points are rarely needed, it is not always clear whether they will in fact derail a runaway train effectively and as safely as possible. For example, use of catch points to derail a train that had passed a signal at danger atLondon Paddington station in June 2016 resulted in the empty train, aClass 165, hitting and severely damaging anoverhead line electrification stanchion, causing all services to and from the station to be halted for hours.[3]
In 2010, in snowy conditions, atCarrbridge, aClass 66 passed a red signal as well as catch points, leading to the train going down the embankment, injuring the two crew on board.[4]
