I cannot say for Spain, but for the Eurostar it has been like that for quite a long time and it is related to the Schengen area border. The checks are made on boarding, so in Paris, Lille, London, Brussels, and Amsterdam before they started refurbishing the station (I am not sure about seasonal destinations but I can’t see how they could avoid checks at some point). It is not an example of increasing security theatre.
Spain has had train security even on long-distance domestic journeys at least since I lived there in 2008. I suspect it's a reaction to the train bombing they had, but don't quote me.
