GMforPowergamers
Legend
it could be... it really depends more on the relationship from DM to players...So honest question. After reading many people's version of sandbox, what if someone were to buy a bunch of adventure paths, create a hub, seed all the rumors for the adventure paths, and then, turn their players' characters loose in in said hub. Then, whatever the players chose they could follow. Sometimes it might lead to following a caravan that is a dragon cult, and other times it might led to the cold and desolate Ten Towns, and still others might lead to a school of magic. Is that sandbox?
if I am playing and I start to find out about the dragon cult and say "cool, I want to join them" and the DM says "Dude, no they are the bad guys for you to fight" then that is railroad. if he says "WTF... these are the badguys..." then figures out what it would mean for me to join them and starts to run it... that is a sandbox. it isn't just choice of adventure (although that is a good first step) but choices IN the adventure.
in general the more curve balls the DM will roll with, the more 'I didn't see that coming' that he allows the more sandbox it is.
just a few things:
the time we made magical nukes
the time we forged an alliance of pelor and vecna
the time we ended up with a PC marrying the half dragon queen of the kobolds
the time I personally took a deadlands game half way across the country, killed a dozen important NPCs, just to come back and find that the 1 PC left behind at the town we started at had become mayor...