Cast iron as a material may not take too kindly to broaching, especially if the wheels are to be broached full depth of the axle hole. I'd be wary of splitting a casting or wheel boss. I assume Tom is replicating some vintage practice.
Expanding on Adrian's suggestion of using a taper reamer - the taper at the end of a regular hand reamer would suffice.
(Hand reamers are easily identified by having a square at the end of the shank. Machine reamers have parallel flutes full length and no square on the shank.)