Definition of "boilermaker"

The sole boilermaker visited the out-depots if necessary, but such occasions were exceptional, engines being worked up to Horsham on suitable duties and changed over as required.

1952 February, J. Pelham Maitland, “Locomotive Working on Sussex Branches Fifty Years Ago”, in Railway Magazine, page 83

the beauteous Asian was […] knocking back boilermakers and their helpers at an astonishing pace.

2006, Thomas Pynchon, Against the Day, Vintage, published 2007, page 597