I would say that the Brompton is the only folder that will fit between the back-to-back seats found on Merseyside / Chester trains.
Its slimmer than a Mezzo, but an old Diblasi is much slimmer than a Bromoton by some way. The problem with most folders is that they increse width as they fold. The older Diblasi are longer but do not have the folding fork so does not fold in a way that doubles its width. (the same way a strida does). Diblasi often fit in to slim space that a Bromptom or a Mezzo will not. Ie Old style Nissian Micra car boot.
Sometime the prodruing stem of a mezzo gets in the way of a lower lip on a luggage space on a trian( like the type shown with the ifmode picture), but i just angle the bike then striaghten it up again. No difficulties.
There are lots of other slim compact bikes that work well on trains. Swifts, birdies,Curves,carry me.......