Given that bogies can be swapped out, metal welded and everything else can be renewed (and should be renewed at sensible intervals) there's no excuse for coaches not to be of that sort of age, and with the likes of the HST engine replacements should give them another 20 to 30 years from that point in time provided the chassis and skin are maintained.
Out of interest, are modern trains all body on chassis as well, or do any use semi-monocoque structures? Or is train performance weight-insensitive enough that it's worth overbuilding for long life and ease of maintenance?
The quick answer is "Yes"
The long answer is that since the early 1970's coaches have been manufactured in a sort of semi-monocoque structure in as much as there is no separate chassis, although there are some structural members incorporated in the underside of the body.
In the UK, British Rail (and its predecessors) used a chassis/body structure up until the end of Mark 1 production in the late 60's. I *think* the early mark 2 coaches onwards were semi-monocoque, but everything since the mark 3 in the early 1970's was definitely of a non-chassis design.
Train performance is definitely weight sensitive. The lighter the train is, the faster you can go with the same power input! But, you have to incorporate safety structures, as well as ensuring that the seats and other things incorporated in the train are robust enough to last for the required lifetime to the first refurbishment. It's no use making a really light coach with extra light seats if you have to send it for refurbishment after 5 years instead of 10.
