Val d'Isere does not have a train station so unfortunately you cannot go directly there with train. However, you can go by train to Bourg-Saint-Maurice, and then from there you take a taxi or some other type of transfer for 40 minutes to reach the village. From Chambery, where the closest airport is, you can go with a direct train to Bourg-Saint-Maurice (not directly from the airport though). You can also go by train all the way from both London and Paris with only one or two changes of train.