I'll be playing a game, it all goes fine for a long time, then suddenly out of nowhere none of my trains can find a route anymore, they all get the x symbol. Try re-routing the broken trains, it's as if the tracks don't exist suddenly, like a flag got switched that turned them all off or something.
Pretty sure this is happening after particularly bad bouts of track-laying lag.
None of this used to happen when I first got this game, not the track laying lag, not the trains suddenly breaking and being incapable of finding routes.