Opportunity to build on a plot of land next to the suburban railway line?

Do not install a decentralized ventilation either, but a central one.

Why is that here?

I doubt that anyone in the house will notice anything of it there, I can hardly imagine vibrations either.

How far do the parents live from the building plot?

Regarding the leasehold issue: That is still unclear, maybe they will gift me the land even earlier.

Should I have praised their cleverness too early?

A weekend camping and sleeping there. If you have no problem with that, then not in the house either.

On weekends, the train runs less frequently, during the week we camp there. Although consistent noise is better than neighbors who chatter nonstop. We are blessed with those. They only come out to our place in the evenings and stay until late at night.

My parents built on a railway embankment. The S-Bahn was on the other side of the railway embankment; on our side, long-distance and freight traffic ran on the 4m high railway embankment. If you sit in the garden, you sometimes have to pause your conversation when freight trains pass by. The S-Bahn passed by and that's fine, it doesn't really disturb. A big advantage for you: the railway embankment is somewhat recessed, which should directly absorb a few decibels. Between 11 p.m. and 5 a.m. on weekdays, not a single train will run - so it doesn’t disturb you when sleeping either. I would rate the advantages of the location higher than the few S-Bahn trains. Controlled residential ventilation and you can always sleep with the window closed, possibly even opt for soundproof windows. Plant dense shrubs facing the railway embankment.

Why is this here?

Because with decentralized systems, noises tend to penetrate inward more strongly. Although there are soundproofing elements, they only reduce the noise to a limited extent. Therefore, I consider a central ventilation system to be the best solution for houses located in noisy areas.

I think if then:

Build with double walls and inside with calcium silicate bricks
Central controlled residential ventilation with air well
Windows with extra soundproofing, also consider this during installation

Then it can really turn out well. And depending on how the area may be developed, place the terrace away from the railway line.