Trains at Idle

[Li'lJugs]

I'll take your word for it, but I still really don't think that's it.  But thanks.

[coborn35]

And I am that modeler/railfan.

 I don't think the sound I am curious about has anything to do with the air system, but of course I may be wrong.

 I am wondering what that "snapping/sparking" sound is?  I usually notice this when an engine/s is making a slow movement, such as switching/whatever.  It sounds, to me, exactly, like a "spark", whether that be an auto spark plug or a Van de Graff generator makes, only much louder, of course.

 To me, it sounds like a "short" somewhere, but I hear it almost all the time, so I really doubt it's that.

It is the wheel coming into contact with a not so even rail joint or something on the track/ scrapping flanges. If you ever have the opportunity, throw a rock or metal object at a freight car wheel, and you will hear the ping/spark sound.

[Li'lJugs]

And I am that modeler/railfan.

 I don't think the sound I am curious about has anything to do with the air system, but of course I may be wrong.

 I am wondering what that "snapping/sparking" sound is?  I usually notice this when an engine/s is making a slow movement, such as switching/whatever.  It sounds, to me, exactly, like a "spark", whether that be an auto spark plug or a Van de Graff generator makes, only much louder, of course.

 To me, it sounds like a "short" somewhere, but I hear it almost all the time, so I really doubt it's that.

[AntonioFP45]

CSX Engineer,

Thanks, you hit the nail on the head on my inquiry. What I've been hearing are "Pop Off" valves.

When I worked on transit buses it was a similar situation. The compressors on 1971-74 series GMC and Flxible buses were supposed to shut off at 120 psi. As the buses got older, the compressor would keep pumping sometimes up to 135 psi and the "pop off" valve would BLAST air out just as loud as an EMD! Sometimes people standing near one of these buses when the valve went off suddenly would get one heck of a "WAKE UP" jolt in their ears[:0] !

UPDATE 2007: Guys, I hope you don't mind my bringing up this thread again.  I wanted to help a modeler next door on the MRR Forum who was asking about air pop off.  I figured it be better for him to read this here since CSXEngineer explained it well. - Thanks!

[csxengineer98]

a spitter valve is right next to the air resovares on the locomotive... any time they detect mosture...they "spit"..the clicking sound you hear... sometimes if they detect alot of humidity in the air..or if they are starting to get dirt in the mechanics...they sometimes will make a blowing sound insted of a clicking sound...but the blowing sound is just the valve staying open for more then just a split second
now a safty "pop off " valve will blow air when the air in the resovars reaches 150psi... it makes a long blowing noise... alot of times you dont hear this becouse of how an air compresser works on a locomotive... normaly...the compress stops "loading" (making compressed air) at around 140 or so PSI and donst come back on untill the air in the main res...drops below 130 PSI i think...(been a while since i went to hogger shcool) but sometimes you might have a unit that the will keep making air..even after the 140PSI is reached..so you need a safty valve to keep from blowing your air system apart from over pressher
a note also...
on any GE dash 8,9 locos... the woop sound you hear befor you hear a thumping sound..is the motor turning on the air compresser...just thought i would share that also..lol
csx engineer

[AntonioFP45]

CSX Engineer,

Thanks, I was going to post a similar question earlier with one additional observation.

I've been train watching & train chasing since the 1970s (I'm 40 now). Managers had no problem with me coming into the big Uceta Yard, in Tampa, on my bicycle to take pictures as long as I behaved (gosh I miss those days!)

Up until the early 90s, whenever I came upon idling locomotives to photograph I would hear the long "tssssshhhhhhhh" from the unit or units every few minutes or so but I never heard "rapid clicks". These were SCL U-Boats, GP40s, SW9s, and occasionally Amtrak SDP40fs. I started hearing the "rapid clicking" in CSX units around the mid 90s. Was some type of upgrade made to the compressor systems that resulted in this?

The reason I ask this is because when New York City transit authrority rebuilt the "Red Bird" subway cars (now retired) back in the early 90s, the air compressors were changed to "rotary" type units that couldn't even be heard and the pleasant old "wump-wump-wump" sound was gone.

[oubliette]

QUOTE: Originally posted by csxengineer98

no...a pop off valve and a spitter valve are 2 differnt things....
csx engineer

Yes you are right but I was trying to simplify the scenario. Also I think we probably have different names for them over here but I think we know what we all mean now. I thank you for your advice as obviously you work on them daily as I do over here in the UK but our terminology maybe slightly different, i.e. bogies/trucks etc. Out of interest then what is the difference between a spitter valve and a pop off valve so I can compare them with what we call them.

Cheers

Rory

[csxengineer98]

no...a pop off valve and a spitter valve are 2 differnt things....
csx engineer

[oubliette]

QUOTE: Originally posted by higssy

Yes, it does sound like a pop-off valve, but is quite rapid 1-2 clicks per second. Seems to me that the build up of air pressure would not be that rapid.

Basically we are all talking about the same things call it what you will, spitter valve, pop-off valve or autodrain valve. The rapid clicks is the air being released in spirts as you would not want a continous stream of air leaking off. The main reason for this is when a loco is idling it will rev up when the compressed air falls below a set rate. Once the air is at a satisfactoy level the loco returns to normal idle and any excess air will leak off. You will notice it more when a loco is shut down. As the pressure drops in the loco any moisture is expelled. You will notice this in a form of little spirts. This will get even slower as all the pressure is lost. Loco's make distinct noises whilst running and even whilst cooling down and you get to know the sounds of each loco and what is normal and what is not.

Cheers

Rory

[Anonymous]

Yes, it does sound like a pop-off valve, but is quite rapid 1-2 clicks per second. Seems to me that the build up of air pressure would not be that rapid.

[csxengineer98]

could have been a spitter valve also... or the safty valve on the main air resovars...they are set to pop off when the main air resovare pressher reaches 150psi
csx engineer

[oubliette]

QUOTE: Originally posted by higssy

Hi All
I was at a small CP Yard near my house, and I was fairly close to 3 coupled up GeeP38's at idle, and there was a clicking noise coming from them and was wondering what it was? It sounded like a compressed air sound or maybe a blow off valve. There was no engineer around, any feedback? Thanks

Air Dryer, dependent on type blows out the moisture in the air line or the auto drain valves again which release air. Can sound like a ticking, popping or indeed a knocking but that is usually large compressed air tanks. In the UK we call the auto air drain valves the fart valves. Because once the air is below a certain pressure it sounds like.....well use your imagination.

Cheers

Rory

[mudchicken]

Air dryer for the compressed air system/ brake system.

[Anonymous]

Hi All
I was at a small CP Yard near my house, and I was fairly close to 3 coupled up GeeP38's at idle, and there was a clicking noise coming from them and was wondering what it was? It sounded like a compressed air sound or maybe a blow off valve. There was no engineer around, any feedback? Thanks