我找了找规则：
A player may have any number of cards in his hand at any time.
这一句解释了8楼说的。手牌是没有限制的。
In the unlikely event that there are no cards left in the deck and there are no discards
(because players are hoarding many cards in their hands), a player cannot draw Train
Car Cards. Instead he may only Claim a Route or draw Destination Ticket cards.
这段话应该就是牛角尖的情况，虽然人家没有提到死循环吧，但是那括号里头说的就是与楼主同一类的人~
