It looks like MSB has the improved suspension model, is this right, and if so, will more old routes get this?
Routes don’t get suspension, trains do. The track unevenness level is what causes the trains the be so bouncy, it’s always been like that
Given the 142 was the only train on Blackpool which had the new suspension physics, and that the 350 specifically lacked it when WCML South, it's clear that the suspension physics are applied to trains, not routes. The physics are of course dependent on the track to come into play, but don't do so unless the train is set up as such.
I can recommend doing one of those short Gemünden to Lohr services in a BR 218... quite fun Nothing for people who get seasick quickly though