Posting for visibility after @Neil's comment. I personally had no idea about this "hack", so I'll be making use of it now.
Essentially, if you tap on after 6 pm, you can travel until 3 am the next day on a "two-hour" ticket. This gives you up to nine hours of travel for the price of two hours. Now you only get the most value out of this if you haven't already tapped on, on the same day; otherwise, you'll be charged a full fare.
This applies to every day of the week; however, seniors and kids under 5 get free weekend travel regardless.
Enjoy!
See also: Free Train Travel before 7am Weekdays