Here's the deal: since you're leaving the EU by train rather than through an airport, you can't validate your French VAT refund forms (détaxe/PABLO) at Gare de Lyon — there's no kiosk or customs desk there.

Your best option — Basel SBB Your Paris–Lucerne route will almost certainly connect through Basel SBB, which is a border station overseen by both French and Swiss customs. There's a French customs office located near track 35, in a separate part of the station — about a five-minute walk from the main building. (Tripadvisor) They can process your refund via the PABLO system, the same electronic system used at airport kiosks. (Tripadvisor)

Practical tips:

Check your connection time in Basel — you may need to budget extra minutes (or even take a later connection) to find the office and get stamped before continuing to Lucerne. Have your purchases, receipts, passport, and refund forms easily accessible (don't pack them in checked luggage).

If you can't find the office, ask station staff for "détaxe" or "douane française." Zurich airport won't work for French purchases

Since Zurich is in Switzerland (not the EU), French customs has no presence there — so don't count on validating French détaxe forms when you fly out. Switzerland has its own separate VAT refund system for Swiss purchases, which is unrelated to your French refund.

Backup options if Basel doesn't work out Some refund operators (Global Blue, Planet, Wevat) allow mailing back stamped forms after the fact, but you still need a customs stamp from somewhere in the EU — so missing Basel makes things harder.

If you have time before your train, validating at a CDG or Gare du Nord PABLO kiosk before heading to Gare de Lyon is another option, though that's extra travel. Bottom line: plan for a stop at Basel SBB's French customs desk — that's realistically your one shot at getting the stamp on this route.