I can remember guys did go out to France, but I don't know for how long. I think you also, but don't quote me on this, you have to complete around 600hrs learning French.
French is quite a hard language to learn. You have to learn the difference between masculine and feminine, past and present.
The best way to learn French, is obviously working with them, hence going to France with Eurostar.
learning from a book or in class room, you get to learn the basics but if you go to France, it all goes out the window.
I'm not sure what standard of French you would have to learn at Eurostar, that would be the first question, and if someone replies from Eurostar on this thread they will tell you.
Also you need learn the slang

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Even though my mums side originate from France and mum and dad still live out there, they can be funny people.
You are normally expected to speak some level of French to French people, other wise they will most probably ignore you...lol.
Someone did say (railway rumour alert), if you spoke English to a French signalman, they got funny, not sure if that's true.
I personally think, it should be like the aviation world. The standard language is in English.
Don't be put off by the language, its actually quite funny sitting there on a train and you can understanding someone's conversation.
If Eurostar is what you want to do, then go for it. Don't let the language or staying in France with a family be a problem.
If I was you, I would maybe look into a few good books and then find a good class, that would be my first step and get some basic lanuguage. Once in the company, you will be working with French crews, and that's where your French will start to pick up.