So I just activated the pagefile again (deactivated it to have more space on HDD), this solved the problem.
The amount of mods could be a part of the problem, but I think in another way than maybe thought: Cause of the amount of mods I have very much different vehicles wich comes in different years. So I will replace all my trains with newer ones. Cause of the amount of new vehicles I replace more often than I would do without mods (sometimes just to bring the train in a new era (from prussia to DRG for example)). When I start in 1850 I have 150 years full of new vehicles, while I started all my older games in 1950 where i only have about 50 years of new vehicles. This means, when i start from 1850 I would replace more often than from 1950.
What i wanted to say is: I think that the replacement function needs such a huge amount of RAM. So the mods are the source of the Problem, but not the Problem itself.