is there any option to travel on an overnight train in a cabin with your partner (of the opposite sex) - everything seems to be indicating a big fat 'NIE'.
It's not a problem if you have sole occupancy of the cabin, or if you travel by couchette. Done it many times, and it's not a problem at all. You just have to make sure that you don't (for instance) book two places in a 3-bed cabin. If you're buying online, you can just say that it's two men travelling - no-one will care if you have the aforementioned sole occupancy of the cabin.