Capacity needs to be matched to demand
I think a balance needs to be struck - not much need of a 6-car from Manulla Junction-Ballina normally (Fridays and Sundays and special events aside), especially if it is being fed from a 3-car train.
However, yes, on days with peak loadings, the full fleet should be out, save any in heavy maintenance (and schedule that for another time). Even the break-down spares should be looked at on those days. However, on those days, its important that capacity be matched to demand - no use in sticking the 6-car train on off-peak or on the wrong route if it means a 3-car will need to be used for a different service.
Separately, with more trains being run, they can individually be smaller.
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