我应该转哪条线？ I guess the context is in the subway station. And image there are several different lines. Now you are in one of the certain stations, there might be two or more lines cross here. Therefore, you might get confused that which line you should change to. You could ask that chinese sentence to the station staff. Yating

我应该换到哪条线

我应该在哪一个地铁站转车？

請問我應該改搭哪一條地鐵呢?

我应该换乘哪条（几号）地铁（线）呢？