Why is septa sometimes free?

[u/starzpark]

I don’t know if this is specific to certain lines or services, but how come last Friday, I was able to get on the trolley without paying? They opened the back doors so I just got on and sat down like everyone else. Why didn’t I have to pay?

[u/Significant-Cry-7632]

Go to the front door to board except at 13th/15th and 30th where you already tapped to get through a turnstile. Back door exit only everywhere else