As far as the game is concerned, you have ended your turn and the next player has been instructed to take theirs. And since the rules say take-backsies are only allowed if... Wait a sec, there are no rules concerning take-backsies in Ticket to Ride. In fact, I've never see a game that allows take-backsies.
It doesn't mean you can't take back the move, but ...
Yes, that is what would happen but that is supposed to be a rare situation as it is not intended for the players to hold all the cards in their hands. The rules cover the situation for when there are no cards at all for players to draw and by extension it also covers the situation when a route is claimed and there are now cards to draw
This would be a very rare situation, and the rulebook does not directly address it.
There are basically 2 options for how to handle it.
Simply leave the 5 face-up cards with them having 3 or more locomotives in the set. This is how the mobile and online versions are implemented.
Follow the rule for a similar situation that is mentioned by the rules:
I had the same question, but having scored 189 in a game a couple of weeks ago, I couldn't believe 195 is the max score.
Edinburgh - Athína gets 21 points with 20 carriages, so I decided to explore possibilities from that route.
I set up the board and checked for the best routes to add.
I came up with this setup:
Points for the routes
Edinburgh - Athína ----...