Re: A LOT depends on your engineer . . .

IIRC, those low-speed boosters were used only when starting a train. There was a cog or latch on the 'Johnson Bar' that controlled the power reverse, and at some point when they were notched up the latch would release and the booster would shut off.  I don't think the booster could be re-engaged while the engine was working, certainly not at any kind of speed over about five mph, so it's pretty unlikely I'd have had to contend with the booster except when getting underway after a stop.  #4449 had a 'modern, high-speed booster' (relatively speaking), and I remember a couple of times when the diseasel helpers pooped out that Doyle had to use the booster to make the hill.  Even then, I think the train was down to only five or ten mph when he cut the booster in.