Bedford St. Johns

[paddylucas]

Hi

I regularly commute from Bedford to Woburn Sands along the MVL. Each time the train (Class 150 or Class 153) departs from Bedford St. Johns there is a loud hissing sound. I am at a loss as to what it is. Moreover, this "hissing" does not occur around other stations along the line. Any ideas as to what this is?

Regards

Patrick

[Beeyar Wunby]

Originally Posted by paddylucas

Any ideas as to what this is?

Hi Patrick. Just been looking at this on youtube.

Sounds rather like the Main Res Tank blowing off.

HTH, BW

[paddylucas]

Originally Posted by Beeyar Wunby

Sounds rather like the Main Res Tank blowing off.

That is what it sounds like too me too but why around that station. Surely you empty the tank to apply the brakes when approaching a station and not when leaving it. Still puzzled but a little less. Thanks.

[pre65]

What sort of brakes do those units have ?

Air pressure, or vacuum ?

If it's air pressure it might be the compressor unloading, based on my knowledge of lorry air brakes.

[Beeyar Wunby]

Originally Posted by paddylucas

That is what it sounds like too me too but why around that station. Surely you empty the tank to apply the brakes when approaching a station and not when leaving it. Still puzzled but a little less. Thanks.

I'm not a DMU man, so this is conjecture based on other stock.

I'm guessing that the tank charges gently whilst the train idles in the station and so is fully topped-up by the time the train is ready to leave. When the train revs up the compressor runs faster but its output isn't needed straight away, therefore the excess blows off.

Maybe.

(We don't have this situation on EMUs. The compressor(s) run only when they sense the MRT pressure drop).

[Beeyar Wunby]

Originally Posted by pre65

What sort of brakes do those units have ?

Air pressure, or vacuum ?

Wiki wasn't much help, but the photograph there shows a Westcode style brake handle, so it's probably EP (electro-pneumatic) brakes.

If it's air pressure it might be the compressor unloading, based on my knowledge of lorry air brakes.

Yes, or something similar.

[paddylucas]

Originally Posted by Beeyar Wunby

I'm guessing that the tank charges gently whilst the train idles in the station and so is fully topped-up by the time the train is ready to leave. When the train revs up the compressor runs faster but its output isn't needed straight away, therefore the excess blows off.

I am not a train man at all (just a childhood passion) but what you say makes sense. Thanks.

[paddylucas]

There is no "hissing" at other stations along the line so maybe we can only hear this at BSJ because of something at track level? At the Bleychley end of BSJ the track curves quite a lot. Al the Bedford end there are a series of points.