The argument between railroad and open world games is a tricky one. I tend to believe the best way to play is somewhere between the two extremes, as with most things in life.
As a GM I strive to avoid railroading, but truly open world gaming either requires tons of work (much of which won't be used), or no work beyond random tables.
I don't personally classify a campaign having some story thread running through it as railroading. Railroading to me is forcing the pcs not only to follow a broadly detailed storyline, but also how they follow it, and yes having invisible 'it actually doesn't matter what you do' plots going on behind the screen. There's nothing worse as a player imo to feel like nothing you do makes any difference - I even had a GM once who had a mechanic that wouldn't allow us to suicide our pcs to get out of his damn 'story'.
Naturally, good GMs are good at covering up the mechanics of the campaign, I've used monsters I expected the pcs to fight at one location in a completely different location, when they missed them first time. They don't know that, so they don't care.
I hate time limit adventures personally, they often go hand in hand with railroading. You MUST go there and do this, and you must do it in this amount of time. Heck, how about you just run my pc for me then?
It's a fine line. I do like a detailed setting and story, truly open settings often lack direction ...