A hostler is an engineer (or in the old days often a fireman) who moves engines around when they are being repaired, serviced, or (with diesels) being made up into consists. A hostler works under the direction of the Master Mechanic or roundhouse foreman.
At one time hostling may have been a craft in itself, but today it is simply a job filled by engineers.
When I hired out, I was just a fireman, but I cubbed with qualified hostlers, and became qualified myself. Hostlers are not engineers, but all engineers are qualified to work as hostlers. / Ron
PS, nowdays, at some locations, by contract the hostling jobs may belong to Mechanical Dept. employees. Being qualified as a hostler does not qualify anyone to work as an engineer. Being qualified as an engineer is a separate issue. Even in the old days they had inside and outside hostling jobs. Inside hostlers were not permitted to move engines outside the confines of the mechanical department's territory. Outside hostlers had to be qualified on the rules and signal indications.