When creating trains for a service that is growing in usage on a consistent basis you can either future proof it (i.e. make it bigger/better than it needs to be right now so that it can survive X years of growth) or you can not future proof it (i.e. make it exactly what's needed right now and a year later you're wishing you could upgrade it). It's the same reason when you buy a computer you think about the next year or two of use rather than your previous year or two of use.
