That was certainly what the Kingstown & Dalkey used (I think the term was "piston carriage"), so I assume Brunel's was similar. (I believe the Dalkey piston carriage wouldn't have been run around the train, so strictly speaking wasn't used like a loco - it went uphill on the front, when the train was under atmospheric power, and came downhill on the back under gravity, when it would presumably have acted as a brake van - but I'm not sure whether running round was necessary on more complex atmospheric lines or how it might have been done, other than by manhandling or horses.)
Wikipedia, incidentally, has an engraving of what looks like a passenger-carrying piston carriage from the Paris-St. Germain atmospheric line (
https://en.wikipedia.org/wiki/Atmospheric_railway, about halfway down), but it's not clear how it would have been used.