So, as I said, there are more choices toward the Sandbox end of the continuum.
There are also more choices towards the scene-framing end of the continuum - which is a different end from both the sandbox and the railroading ends - which is a reason to think that it's not a continuum!
Scene-framing is actually an element of Storytelling games that some modern RPGs add-on, which is fine, but it it is beyond the scope of the Sandbox-Railroad continuum.
But it's not beyond the scope of the scene-framing/railroad continuum. As a self-conscious approach to RPGing, it was articulated as an alternative to 90s-era railroading (associated with White Wolf games and also 2nd ed AD&D).
If railroading lies at the end of two different continuums, that's a reason to think that the best framework for analysing the phenomenon is not a continuum.
RPGs, at their root, have players making choices through their characters to execute actions within a setting as controled by a GM (DM, Referee, Facilitator, etal).
Traditionally, players control what their characters try to do, GMs control everything else.
I don't think it's traditional that the GM controls everything else - for instance, I don't think there is any tradition that the GM in 1st ed AD&D is free to ignore the result of a player's attack roll, or the players' initiative roll, just because s/he doesn't like it.
The idea that the GM can override any aspect of the player-side action resolution mechanics has its origin sometime in the 80s (Dragonlance is often cited), and flourishes in the 90s.
Scene-framing is one of many story-gamey situations when the players control things outside their character.
That's not what I'm talking about. I'm talking about the sort of approach to RPGing supported by RPGs like Burning Wheel, HeroWars/Quest, Maelstrom Storytelling and other similar games (I would put 4e among them) where scene-framing authority is in the hands of the GM.
My point is that such games were invented because the inventors (i) wanted the sort of RPG-generates-story experience that mainstream 90s games promised, and (ii) didn't want the railroading that mainstream 90s games were using to realise that promise.
This is an approach to avoiding railroading that does not involve sandboxing. It's a real thing, that some real RPGers actually do.