Since this is a forum about travel by Eurail and Interrail, knowledge about such questions will be limited. It's a question related to immigration, not to train travel. I suppose you'd need to check UK requirements on which passport you should use. I'm not sure the UK does exit checks though, so what passport you use to enter France may not really matter from a UK point of view.
Do use your Australian passport for your Eurail pass. This is however entirely separate from the passport(s) you use to enter a country.
Thanks. I have my Eurail pass on my Australian passport. I think as long as I have both passports with me, that is fine. And seat reservations identification is different to going through immigration.
Cheers
Jon
For the purposes of your rail pass, the passport is only there as ID. Pick either passport, it doesn’t matter. You then need to show that passport as ID if a conductor wants to see it.
The passport you show to immigration doesn’t have to match the passport you have on your pass. As @rvdborgt says, that’s a matter for immigration and probably not something we can advise on.
Bear in mind that some countries require you to carry ID at all times, and they would probably expect that to be the passport you used to enter the country.