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[quote user="Semper Vaporo"] The steam engine works by using steam that is created in a confined space (the boiler) and letting that steam into a cylinder that has a piston in it that can move due to the pressure of the steam.  That moving piston is connected to the main rod to turn the wheels 1/2 turn.  When the piston has reached the end of its travel, there is still pressure in the steam and in order for the piston to return to the other end (and the wheel to continue to rotate) the pressure of that steam must be released.  The steam that is released is put to one more use after most of the pressure has been used in the cylinder to move the piston.  That steam passes though some pipes to the bottom of the smoke box directly below the chimney.   There a pipe, appropriately known as the "Blast Pipe" aims the steam directly up the chimney.  The moving steam in the chimney creates a draft of air going up the chimney. The air that is moved up the chimney has to come from somewhere and that is through the flue tubes in the boiler.  That air has  to come from someplace and that is from the firebox at the other end of the boiler.  And of course, that air has to come from somplace and that is from the openings in the grate on bottom of the firebox where the fuel is being burned.  The fresh air being drawn in that way helps the fire burn hotter which creates more steam to be used to make the pistons move and thus move the locomotive. Anyway, it is that sudden release of steam up the chimney that makes the chuffing sound.  On the most common configuration of a locomotive there are FOUR chuffs per revolution of the wheels.  There are two pistons, one on each side of the locomotive, and each piston is alternately pushed to one end by steam and when the piston reverses direction and pushed from the other end by steam, the steam is released, creating a chuff.  You hear a chuff from the front end of one cylinder, then a chuff from the front end of the other cylinder, then a chuff from the rear of the first cylinder and then a chuff from the rear of the other cylinder.  At that point the first cylinder is already moving due to pressure in the front of it and the next chuff is from that end of that cylinder, which is the cycle starting all over again. You will note that the pistons do not move in the same direction all of the time, nor do they move in opposite directions all of the time.  When one piston is just starting to move one direction the other piston is in the middle of its travel in the same direction.  When the 2nd one reaches the end of its travel and reverses direction the first one is in the middle of its travel.  When the 1st one is reaching the end of its travel, the 2nd one is again in the middle of its travel but in the opposite direction. So half the time they are traveling in the same direction and half the time they are traveling in the opposite directions. Anyway, it is the release of the "used" steam up the chimney that creates the CHUFF sound.  When the throttle is open all the way to get the train moving is when the chuff is loudest.  Once the train is moving, the  throttle is closed a small amount (or more importantly the valve gear that times letting the steam into the cylinder and letting it out is changed to let less steam in) then the chuffing quiets down.  Thus starting the train, acceleration or working up a hill makes louder chuffs than when just holding speed, and the chuffing can almost go away when slowing down. There is a lot more to this, but it takes huge books to explain it all and the various differences that have been invented to make the locomotive more effecient or easier to maintain. if you really, really want to understand all of this and like to read, I recommend the book "The Locomotive Up To Date" by Chas McShane.  Get the 1909 edition!  There are a couple of publishers that have reprinted it and it is not too expensive, but it contains some really good explanations.  There are some more modern books, but the McShane book is a classic and can hardly be outdone. [/quote] This is one of the best posts that I have read in Train's forums.  I am not an engineer, but I am able to understand what you have said.  Also, many of the responses are equally informative.
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