This is from a couple of forum posts:
The carriage is the train you see at the left side of the game screen.
Carriages are added after removing a certain amount of cards in a row ( I think it’s 5 cards) before having to click on the draw pile.
You can check for yourself by watching the train move when you remove 5 or more cards in a row before having to click on the draw pile.
There will be a number on the carriage showing how many cards you removed.
The credit we receive toward the challenge is based on how many times the train advances during that round
You will get credit for one carriage each time the train advances.
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