Two hours should be enough time for passengers to travel Taipei to kaohsiung by high speed rail,____ 為何込Isn't it 不是aren't they?

Two hours "is" an amount of time. That amount to time "is" enough for any passenger to travel from Taipei to Kaohsiung by high speed rail.
通常附加尾句的主詞必須與主句一致。
two hours在形式上雖為複數，但實質上是「一個單位」，所以用單數代名詞it來代替它：
Two hours should be enough time for passengers to travel from Taipei to Kaohsiung by high speed rail, isn't it?

主句在be動詞前用了助動詞should是因為所述是「我個人的」主觀判斷，但「附加問句」是在邀請聽者認同，所以問者的立場必須回到客觀的狀況，就不宜再用助動詞should了（在原主句的主詞是We的狀況下，主句用了助動詞should，附加問句才有可能再使用should）。

請參閱柯旗化《新英文法》p.612：
單數性質的複數名詞＋單數動詞：
the United States + is/does等
two thousand miles + is/does等
ten thousand dollars + is/does等
eight hours + is/does等

The subject is singular when it refers to one person, thing, group
The subject group "two hours"is singular in Grammar;however long,whatever the no. of passengers;
=isn't it.

Two hours should be enough to travel from TPE to KHH by HSR, isn't it? (X)
Two hours are enough to travel from TPE to KHH by HSR, aren't they? (O)
One hour is not enough to travel from TPE to KHH by HSR,is it? (O)
Two hours should be enough to travel from TPE to KHH by HSR, shouldn't they? (O)

Two hours should be enough time for passengers to travel *from Taipei to kaohsiung by high speed rail, isn't it?

乘客們 坐兩個小時的高鐵 應該足以 從台北遊覽至高雄, 不是嗎？


P.S. 用單數 「isn't it? 」是因為 這是講 關於「乘客坐高鐵遊覽的事情, 而不是計算在乘客🈶️多少...」^_^