Hi Thorsten,
Sorry for the hugely delayed answer!
You are so right about "nicely aged"
I actually managed to take off the 6 wheels on the motors without breaking anything...and found what's probably 30 year old hairs tightly filtered around the axles. Removing the hairs did some good for the power of the motors and now I have ordered some IPA and will give the rails a careful treatment. Depending on the result I will bring in WD40 as you say.
I'm a bit surprised how well the two trains run. When not having used them for a couple of days they need to do 1 or 2 laps where they might stop and need a small nudge, but after that they have "cleaned" the rails and after 10-15 laps they are up to pretty much max speed from what I can remember. Even my stop signal works and can stop the train and also the railway crossing works and can lift and lower the boom. The only parts that haven't really survived 30 years in plastic bags are the "motors" for my two track switches (or what they are called in English). They send out a constant electrical humming which isn't nice to listen to.
It would be very nice if I could somehow replace the 12V train motor contacts, as they are somewhat worn, but from what I gather that's not really an option without it becoming are very DIY project.
Best,
Morten