Our current experience reflects the campaign to be sandbox, but the Adventure Paths that are merged within the campaign, which has other player-driven story tangents as well, follow a predominant linear design, but not entirely as it depends on the AP.
Are there incidents of railroad, for sure, I can think of a specific series of railroad situations that I engineered because of an idea I quite liked online which had fleshed out more of the AP for play.
In SKT the party is meant to go to one giant leader, obtain the teleportation conch and travel to the Storm King's palace.
That leaves massive sections of the AP unexplored with beautiful content material (setting, personalities, relationships, locales etc) wasted. The idea I found online assisted through the various NPCs and certain character declarations to ensure the PCs visit more than 1 of these giant leaders in an effort to travel to the Storm King's palace.
I have subsequently told the players of what I did, but they did not mind - they enjoyed the storylines that arose, the in-game forces felt natural to them, their PCs benefited in the overall story for investigating these other locations and as an aside they have a lot of freedom within the campaign itself.
I think with this incident, it is difficult for me to claim that all railroading is bad.
As PCs increase in level, mechanically the game provides them greater control of the story which can emerge.