Ok it's that the user group also has to be root. I ran the following:
sudo chmod 775 -R /opt/trains/ sudo chown -R root:root /opt/trainsand it works.

It seems that it has to be 775 with both user and group as root. E.g. 771 does not work, because than the docker command has to be used with sudo (if I want to use my default sudo-user account)