OK ... background to why I asked the question.
In my youth, train travel comprised suburban commuter runs in south London / north west Kent, and long distance trips - typically with family. We would settle into our long distance train at Euston, King's Cross, St Pancras or (occasionally) Paddington along with other passengers, and most of us would remain on that train for many, many hours. Journeys were long, stops were infrequent, and passengers leaving / joining at those intermediate stops were few.
In those days, I can recall walking through the train to the buffet car for cups of coffee, perhaps the odd snack, and the proverbial "British Rail" Sandwich and carrying them back to my party through up to 10 coaches that bucketed and rocked over junctions along the way. But these days, my journeys have changed. For sure, some are long distance; I'm in Scotland later this seek, for example. However, gone are the long times between stops and gone for many of us are the larger family group. Intermediate stops see huge flows of people off and onto the train, and it's no longer the case that those who are seated opposite you as you leave London and the same people who get off the train in front of you in Edinburgh, Liverpool or Minehead. These days, I am reluctant to leave my seat. I will do so for a couple of minutes to visit the loo (though even there I'll tend to wait to virtually the end of the journey in case I loose my seat, and I'll take my personal stuff with me; much of that comes from travelling alone, with the rest of it being from knowing some of what goes on, having seen the aftermath of someone having their bag taken off at an intermediate station without their knowledge, and from concern of not being able to get seated again.
So ... the "longest runs" question relates to buffet car logic.
And as for all this rubbish about a buffet car, you don't need a buffet car, most people prefer at seat catering these days, otherwise wouldn't airlines be copying.
Hmmm ... I don't think a buffet car on an airline was ever feasible, though I do know that on some airlines, first class offers a bar area away from your seat; great to stretch out, etc ... and little danger of losing your place or luggage at an intermediate stop.
I do note that some trains have trolley service and a hot food preparation / special order area where the chap (or chapess) on catering can go and bring you your order at seat through the train. Perhaps there's sense in the expansion of this - using the train's WiFi and intranet to allow passenger to place their orders. Even if the train's in a dead zone, this would be self-contained, and being an intranet it would avoid you ordering and paying for food that ends up being delivered to the same seat on the train that;s immediately behind yours!