While working on the PRR(by then the PC) in 1968 I was called to protect the job of head brakeman on a extra.When we sign in we was told we will be taking some cars of rail to a minor derailment site and we are to drop the cars and return light to the yard and pick up some ballast cars and take those to MP22.3(?) and leave them on the west bound main and the crew assigned to wreck duty will pick them up when needed and take them on to the wreck site.Sounds like another simple day and rather boring. The conductor told me "Go get the engine boy and couple up to those cars of rail".The unit assign to us was a RSD15 in the 6800 series(the PRR had 6 RSD15s numbers 6811-6816) so we picked up the unit proceed to couple on the train and pump up the air if memory serves me right we had around 10 cars of rail.We was just clearing the Columbus Union Station area and was about to clear the High Street overpass when the Alco made a coughing noise-for the lack of a better word and smoke bellow out of the stack and from the long hood of course we thought we had another Alco on fire so the engineer stopped and went to look and could not find the source of the problem causing all that smoke and of course there was no alarm bells ringing..Of course we radioed the problem in and was told to proceed on as that problem had been reported by the last engineer and not to worry about it..(Thanks for telling us ) So we proceeded on our way and the smoke really begin rolling out of that unit so once again we stopped and looked for the cause and again found nothing amiss.So once again we continue on our not so merry way. When we arrived at the wreck site to drop the cars off the Division Superintendent came walking up the track and proceeds to ask the engineer what the hey(not his real word) is wrong with that unit? I seen you guys coming for 3 miles and I thought they found and fired up a steam locomotive! The conductor and engineer told him what was told to us and needless to say his face got red and he started cussing and told us in no uncertain terms to take that blankly blank engine back to St.Clair(the shop area) and leave it there and I will deal with the lunatic that told you to use that engine.. When we got back to the Cleveland Avenue yard we was told to take that unit to the shops and pick up unit number XXXX (sorry I don't recall the number anyway it was a GP9).We then finish the days work without future problems...I did find out later that the turbo charger was blown.