counted many rivets lately! It's a charming garden railway, just enjoy the fact we've had the chance to enjoy seeing it

What comes out of the chimney is a mixture of exhausted steam/water vapour from the cylinders (the puffs), and an amount of smoke/air from the fire in the firebox. The "puff" is caused by the exhaust valve in a cylinder releasing the steam at either end of its stroke, and the part you can see is water vapour, as steam is invisible. A steam locomotive will usually have two or three cylinders, and each will exhaust steam when it reaches either end of the stroke. The ferocity of the puff will vary based on how hard the locomotive is working, with the loudest being produced when the driver has the "reverser" fully forward/back (which means steam is allowed into each cylinder for a longer portion of its stroke, and hence is higher in pressure when it is released at the end of the stroke), and has the throttle (or "regulator") fully open. It is important for the exhaust steam to be piped into the smokebox (the black bit with the door at the front) and up the chimney (rather than just exhausting into the air from where the cylinders are) because this is one way in which the draught is pulled on the fire to make it hotter and to suck air through it; The other is a system of deliberate jets of steam (called the "blower"), which is used to induce a similar draught on the fire when the locomotive is not moving (or is not working hard enough for the exhaust beats to do this effectively). If there is no draught from either source, the fire will be starved of air, and if there is a lot of coal burning in the firebox, this will cause the smoke to turn sooty and black (indicating incomplete combustion of the coal). A good fire will produce a light grey smoke in between the puffs of water vapour.

@@C.I... wow!!! You have done excellently with the explanations of the dynamics of the steam engine sound as I requested. Well done