Word on the train/scheduling:
"Generally, trains in France can be booked once within 90 days of an intended date of departure. However, with regard to the direct trains between Paris and Reims or Champagne-Ardenne, the national rail network of France has not yet enabled ticket purchases for dates after July 2nd. We'd expect them to make these tickets available very soon, probably within the next 2 or 3 weeks. Until they do, you can use an earlier date, some time in May or June, in order to get an idea of the ticket costs and schedules for trains on this route. You'll be able to purchase your tickets once the direct trains display for the actual date you'll be traveling. "