non-trailable point machine

[Amrutha]

Can you please clarify the difference between trailable and non-trailable point machine?

[PJW]

Can you please clarify the difference between trailable and non-trailable point machine?

In the UK we always (with very few exceptions) use non-trailable machines. The two switch rails are connected to each other by stretchers and there is one drive from the machine and a form of Facing Point Lock to hold firmly in position. This also means that if a train attempts to pass through trailing points that are set incorrectly that it will encounter switches held firmly; this will result in damage to machine (perhaps), rods and other sacrificial components (definitely) and P'Way switch itself (quite likely). The train could also be derailed, but generally it is the point machine that gives (frangible links provided for this reason).
For other forms of Point Operating Equipment things are similar. The clamp in the Clamplock mechanism distorts from the extreme pressure of a train forcing its way through points set incorrectly. The latest In Bearer Clamp Locks feature SO backdrives incorporating locking; specific Break Out Devices are used to permit the point to be trailed without too much damage (but this isn't the same as being "trailable"!). A perceived problem with the High Performance Switch System is the fact that they may hold the switches so firmly in position that a train may well be derailed rather than the switches forced open.

Many railways (particularly those not having a UK railway heritage) use trailable switches- each switch blade separately driven and detected from the machine, no stretchers, no facing point lock. When such a machine is trailed with points set incorrectly, the train tends to force the points across (hopefully without sustaining damage). From that perspective obviously there is an advantage in that the time it takes to get the railway operating again after such an incident. The UK approach probably dates from certain accidents well over a 100 years ago and, having effectively got incorporated into law, it is not so easy to change even if we wanted to. See recent IRSE News article. Obviously the lack of trailability of running line points only becomes evident following an unauthorised train movement so the best approach is to attempt to stop these happening in the first place. The fact that damage is virtually certain to occur at any trailing incident in a strange way can be an advantage; it becomes obvious to the signaller since there is a loss of detection, so a technician will be sent to investigate and rectify- otherwise there would be a chance of damage occuring but remaining undetected and thus creating an ongoing safety risk.

The only points on NR that are trailable are handpoints in sidings, spring points (not modern practice but still exist particularly as traps entering loops or sidings) and Train Operated Points on remote lightly used lines that are delivberately designed to be trailed through but then motor back to their Normal position. There is a severe speed restriction over them. No trailable point machines as such.


Investigate the folling links for more info on a selection of different type of machines:

http://transport.alstom.co.za/products_s...ipment.htm

http://www.westsig.co.uk/assets/files/Do...PM1-DS.pdf

http://www.railway-technology.com/contra...ions1.html

http://aunz.siemens.com/Mobility/RI/Docu...ochure.pdf

http://www.patilgroup.com/point-machine.html

http://findarticles.com/p/articles/mi_m0..._58673324/

http://findarticles.com/p/articles/mi_m0..._58673324/