score:12
Here is how it is supposed work: in the station (Hong Kong or Kowloon) you go through a turnstile. The turnstile has an RF card reader which can read (and invalidate) the ticket. Typically you need to swipe the ticket over the reading area next to the turnstile but if you are lucky the reader may still see it if it's just in your hand.
After it's read, the turnstile will unlock and you can go through. At this point the ticket is invalidated (or one trip subtracted). You are supposed to keep the ticket since it's also your receipt.
So if you went through a turnstile, your ticket got read and invalidated. If not, you got lucky and have a spare ticket. Same story as if the conductor on the train forgot to punch your ticket. It's not your fault.
Upvote:3
There is no crime in taking the ticket. A ticket can often be useful as a receipt for business expenses purposes, especially if you didn't get a receipt when you bought it. I'm not sure how you managed to get to the train without going through a ticket barrier in Hong Kong though. In any case you paid for the journey and have a ticket to prove it.