Series or parallel on the batteries?
Are you thinking of a 'standard' package battery, or something more exotic?
What is the charge density (or mA-hour rating) of your suggested batteries?
If you put them in parallel, you may well find as the charge/voltage drops-off, the loco ends up running too slowly too quickly..
If you put them in series, you might be limited by the current you can draw. - If (say) using a small standard package battery.