You can't buy a train ticket, you can only buy a plane ticket.

1 First of all, you can express your inner sense of loss when talking about this matter, for example, you want to buy a train ticket but you can't.

Secondly, I expressed my helplessness in the conversation. For example, air tickets are so expensive. I wanted to buy a train ticket instead of a plane ticket, but I was helpless.

3. Finally, sum up two moods.