Hey there,
I`d like to start my France trip in Cologne; the operating train service is french (TGV/Thalys), is the country pass valid from here? I found mixed info; the pass only makes sense for me if I know upfront of my trip whether it`s vaild. Seems to me that validation in border regions is very up to each country, so I`m very happy for the help of someone with experience with the french one country pass 
Thanks so much!
No on Thalys you need a Global Pass or an One Country Pass BeNeLux.
And surcharge for TLS is so high that it nearly equals the lowest priced advance fare-but you wont get that low fare in hi-season.
For IN DE there is this summer the 9€ monthly-for LOCAL transit only.
If you take a regional train across the German/French border you can use the 9 EUR ticket to the border and your One Country French pass from the border.
https://www.bahn.com/en/offers/regional/9-euro-ticket-en
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