On the Eurostar? I don't think so.

If you have a UK, France or Belgium address, you can join the Eurostar Frequent Traveller program; EFT points can be redeemed for travel on United Airlines. (Note that their redemption schedule is structured somewhat differently from Mileage Plus'.)

You can earn Mileage Plus miles by mailing copies of your ticket stubs to the Mileage Plus Service Center.

I don't see any reason one couldn't accrue credits in both programs simultaneously.

SNCF travel between Paris(CDG) and Angers(QXG), Bordeaux(ZFQ), Le Mans(ZLN), Lille(XDB), Lyon(XYD), Nantes(QJZ), Poitiers(XOP), Rennes(ZFJ), or Tours(XSH) can be booked with a UA flight number.