Hello Alexander!
It would be quite easy to just blame the mods you have installed, but I have to point out that I very rarely saw Train Fever using such amounts of memory without any mods.
To double check: Would it be possible for you, to play a mod free game and see if the behavior changes?
Also it would be worth having a look at the mods which trigger around the mentioned timeframe and try without them.
Best regards,
Tom