(Not much oxides of nitrogen though.)
Yes the switch to diesel received universal accolades for being clean!
This locomotive never burned wood, and could not. The tip-off is the smoke stack, which is so small and short it is almost invisible. Wood burners require large, fat stacks that hold the screens that filter out the burning embers--without which you would be setting grass and forest fires all along your route as you chugged merrily along.
The thick smoke shows the fire in the box has been freshly stoked. As the coals burn down the smoke they put out can decline nearly to invisibility (in a photo like this. It never really falls to nothing.) The Fates are kind.
This is not entirely correct. First, the smokestack of this locomotive is not small: most of it is inside the part in front of the boiler (the smokebox), which is much bigger than on the popular older American locomotives. Second, newer spark arresters didn't require as much space, and were built inside the smokebox. Third, spark arresters were required in Europe for coal-burners too, especially on burners of low-quality coal - and even they don't stop all sparks, that's why nostalgic trains are prohibited to run during summer droughts here. See the cut-view of German locomotive class 86 - the spark arrester is marked 107:
BTW, wood was burnt in coal-burners sometimes (at least it happened in some instances in post-war Europe.
As for freshly stoked, thanks, I never thought of that (as an excuse, I'm too young (and not enough involved with nostalgic trains) to know steam locomotive operation well enough). *Lunatic*, n. One whose delusions are out of fashion.
