Class 172 Question

[clarkkent]

Hi folks

Living in the West Midlands I have daily experiences with LM's 172's and there's something that I've wondered ever since my first ride on one and this is probably a very silly question; when pulling away from standstill after a few seconds there's what sounds like a double hiss/air release sound from the cab. This seems to coincide with an increase in power after this hiss has happened.

Do any of you knowledgable train folk know what this sound is and it's function??

Thank you in advance!!!

 

[bronzeonion]

I think it's the door interlock for the cab door as it's audible on all 'Star units.

 

[slick]

https://www.youtube.com/watch?v=LeJq7kx4usI

Same sound at 0:12?

I'm not entirely sure if 172s share commonalities with the electrostar family as bombardier built units but on the trains I drive (379 emus) that sound I believe is related to the door interlock and the low speed sensor engaging which is usually around 3-4mph unit dependant.

One of its purposes is to interact with the egress. You can pull the egress at 80 mph and the lever will only pull down to its first notch and the door will open with a small gap of maybe an inch or two. An emergency brake application will follow and only after the sensor disengages around 3mph will you be able to pull the handle fully down and pull the doors all the way open to exit the train. That way you cant fall out of a train moving at any great speed. On older units its a distinctive clunk or click. On older untis ie..on a 317 if someone were to pull open a door below 3mph before the low speed sensor engaged you would loose interlock but the emergency brakes would not apply and the train would still roll, but above the engagement speed it will apply the emergency brake if interlock is lost. You hear it disengage when coming to a stand during deceleration also.

Not sure what other criteria it covers as they dont go into it too much in traction but im sure someone else can give lots of other systems it and equipment it monitors and interacts with. Not sure why on bombardier stock it sounds like a puff of air though, different equipment I guess.