@Willow Mist. Not positive about the train coupons, but I don’t believe that there is a limit n how many you can accumulate. There was a time when I had 24 coupons for train holds. Gems , are stored in the laboratory and can accumulate as many as you want!
Just to add to what @Gracie said @Willow Mist I don't believe there is a limit. I saw a screenshot from a hopper (when hopping was still a thing) showing they had 490 coupons to fill train/plane holds. Is this the coupons you mean?
But for normal folk like me, I would say you are rich with 101 coupons!