particle の - Meaning of trailing の in a question

I've been running into this trailing の particle quite often and can't seem to understand what exactly it means. Is it replacing か, or does it mean something else?

For example:

まだ生{い}きているの？

Answer

"Instead of 「か」, real questions in casual speech are usually asked with the explanatory の particle or nothing at all except for a rise in intonation"

http://www.guidetojapanese.org/learn/grammar/question