Paul Willis
Hi Jonathan,
The answer to that question would be “what is the prototype that you are modelling?”. That will then dictate what the correct spacing would be.
I recall that the EMGS track was based on fairly modern 60ft track panels with a spacing to match that. Other track may based on different prototypes, or simply “generic”.
Sleeper spacing and track panel length is more a function of period than purpose. Rails were initially unable to be produced in lengths of more than 30ft. I think that the LNWR were the first to produce 45ft rails, and technology went on from there. Whether a piece of track is more mainline or branchline would be chiefly indicated by the weight of the rail (and thus the size of locomotives that can be borne) rather than anything else.
Best,
Paul