Bob and I would be referring to a nine inch swing lathe (9" is the largest diameter the lathe can turn across the bedways). The length of the lathe's bed is not that important for driver work which is generally done close to the headstock. Before I had my 9" Southbend I machined drivers on a 6" Atlas lathe with a bed of approximately 18". So a 7" can certainly do the job but perhaps at a slower pace due to its smaller size and rigidity.
I also have an old (1937 vintage) 13" swing Southbend with a 1 HP motor that is primarily used to cut driver tire blanks from heavy wall steel tube or lead alloy steel bar stock once it has been drilled and bored out to size. A standard cutoff tool is used running the lathe at about 60 rpm to cut off the blanks. The blanks are then fitted to the driver castings, half of them insulated with 0.010" "fish paper" insulation and finished to size and profile on a special mandrel held in a lathe collet on the 9" lathe to maintain concentricity.
Exact replacement driver castings are not readily available for the diecast hi rail models. When House of Duddy was still around I would purchase their machined GG-1 drivers for conversions when I could get them. Other than those, I found it easier to remachine the hi rail drivers. I have machined thousands of steam loco drivers at this point in time and now remachine the Lionel, WIlliams, Weaver and MTH GG-1 drivers as well.
Joe Foehrkolb