@tanterei yep, that is why I am saying it is probably closer to po than choo. Well, that and the fact that choo and other high-frequency sounds is hard to make without much smaller cross-section, so even if a build-up occurs (puffs of smoke rather than continuous smoke) it would not become a choo.
Also, I highly doubt there is compressed steam (or uncompressed for that matter) in the chimney like you claim. Most likely that smokestack is dedicated to the smoke produced by burning coal, since it is black.