At this link it says
http://www.freefrequentflyermiles.com/index.htm
"United usually offers free train travel on SNCF French Rail, France's high speed rail service, when you fly to France on United. Read the article to find out how to get it. I believe the train trips are code shared with United, so you probably have to buy your ticket from North America to the final train destination. But I don't know how to get the "train coupon". Ask United about all of this."
So possibly United miles could be used for these trips?