Ok so I am now certain that the last substantial town with the impressive fort on top was Briançon and so the border crossing was Montgenevre / Claviere. The other (much smaller) town with the train station and the gendermerie about 90 kilometers from the border is lost in the mist of time. Could by Embrun, Chorges or a score of other little towns. If any of the helpful commentators wishes to turn his comment as an answer I’ll gladly accept it. Thanks for helping me find Briancon!
Credit:stackoverflow.com‘