This particular photo appears to have been "staged" for maximum soot output.  This specific train only ran a couple of excursion routes per year in the late 1990s and is not in regular operation.  Below are a couple of other pictures of this same locomotive passing over the same viaduct:

So it's fairly obvious that this locomotive isn't always such a polluter.  It's not terribly clean, no doubt, but 614 was built in 1948, just prior to the main switchover from coal to diesel-electric.