Head end power is one major difference between a freight engine and a passenger engine today. The other is gearing.
However, back in the dark ages (?!), many lines used various B-B diesels as passenger engines, for a variety of reasons. Sometimes they needed the tractive effort (mountain railroads), sometimes they just didn’t need a whole raft of special engines. In either case, a steam generator or generators were fitted to supply steam to the cars. The most obvious examples (from a spotting standpoint) were the GP-7 and GP-9s. In virtually ever case I know of, the ones fitted with steam generators carried the water in extra tanks under the frame, where the air tanks usually were, and the air tanks were relocated over the long hood. They were referred to, commonly, as ‘torpedo boats’ from a certain resemblance. The steam generator lived in the short hood. Also, at least on the ones I was familiar with (CV had a number of them), no dynamics were fitted. The gearing was sort of a compromise. A number of Alco RS-3s and RS-11s were also fitted with steam generators for the CN and CV.
The Trainmasters were excellent commuter engines. They loaded really fast, which is what you want in commuter stop/start type service.
Another thought on the Algoma Central, now CN. The AC used geeps in addition to the F units. I’m not certain but thnk they had a separate power care for trainline heat/electric.
There were two flavours of Geeps on CN and its family lines (CV had them too) – one with steam generators (you can identify them – they are the ‘torpedo boat’ style); the steam generators (pesky mean nasty minded things they were, too) were in the short hood. Many Geeps did not have steam generators, though. Originally, the torpedo Geeps were the only ones used for passenger service, but with the end of a lot of passenger service, a lot of the steam generators were removed or at least not maintained. Then later you got passenger cars which needed head end power. Either way, if you were using a Geep, you needed a power car (not uncommon to see a very elderly F7B with steam generator in that function). It could be set up for steam, or to provide head end power – or both. The same principle is used for steam powered excursions…