Hello RS Fans and tour members; According to RS France Guidebook and to our Eastern France Tour documents/tips/info etc, it appears that express trains Paris-Reims (or Reims-Paris) are only into/out of Gare de L'Est. However, there are also express trains, only slightly longer duration (just about 10 minutes) into/out of Gare de Lyon. So when searching for a train that fits your timetable, be sure to put "Paris" in the "from" (or "to") bar, rather than specifying a station, so that you will see both/all options.
We are spending a couple days in Paris, pre-tour, and then looked for a morning express train to Reims on Sunday 10/11 (starting day of the tour). It appears that morning express trains go into and out of Gare de Lyon, while afternoon express trains go into /out of Gare de L'est. Hope that is helpful info for someone out there!
Might also post this under "Transportation."
Nancy NH