we went out to the alps with Eurostar in the summer for some riding. Getting out was fine, the bike bags went through the xray machine and we were given some help getting them on the train.
On the way back we ended up in Paris, tried to check in and a very grumpy french lady demanded that we pay 50 euros per bag, or we would not be allowed to travel. Luckily two of our group speak very good french, and were able to argue the toss about it. We missed our train, paid 20 euros each, we told we would have help loading them on the next one, got none, wrote to Eurostar when we got home, got a full refund and the offer of half-price tickets.
The moral is this: phone them, check the _current_ rulings on bike bags (the french and english versions of the websites for Eurostar were contradictory when we travelled), get the name of the person you speak to, and ask them to email you the information, print the email, carry it with you.
Once in Paris we didnt have any trouble with putting bike bags in Taxis