That could (does, it seems) work for planes, where there are limited entrances, everyone has an allocated seat and staff have control over passengers. None of those factors apply to trains, but there must be scope for thinking about luggage racks – ease of access, positioning and number – to make them quicker and easier to load in a way that allows others past. Also doors and their steps.
...except if they do? There's plenty of trains in the world with all seats reserved, and controlled access to platforms, where baording could be phased. Even with current procedures and gate equipment
SNCF▸ could do it where two
TGVs▸ are coupled but given separate train numbers. For the biggest loading surge, at the start in Paris, you keep everyone waiting on the concourse and call the front train first. Perhaps they already do that...
But in any case I've seen that news item several times before. Airlines think about it every time the subject comes up in a management meeting (and does implementing it have anything to do with Gatwick?) In practice they can't be bothered, or in the case of Easyjet do the opposite and stop allocating seats altogether.