What is the proper term for the "chugging" of a train?

Question

What is the proper term for the "chugging" of a train?

Answer 1

A steam locomotive is said to "chug" or to "chuff."  If you want to sound scientific, you can call it the "exhaust note," or just the "exhaust."  "Bark" is sometimes used as a slang expression.

Answer 2

It is the "exhaust."

The emissions from the burning of fuel creates some smoke (but this is the job of the fireman to minimize) and it, as well as spent steam exhausting from the cylinders, is exhausted through the smoke stack via the "smoke box," which is right behind the cylindrical portion of the boiler at the front.

This produces the familiar chug, chug, chug.  A slang term for this sound used amongst rails was "barking."

The other hissing, whirring, thumping and "pounding" sounds come from either cylinder-c***s, the dynamo, the injectors, the blower, stoker (on coal burning locomotives), damper operation and steam driven air pumps.

When the fireman operates the "blow down," steam is exhausted directly from the boiler.  It is extremely loud and usually not done when there is a crowd around, but always at the end of a run.  This helps prevent build-up of sediment or other mineral deposits from the water.