Once A Fool
But a sandbox can have villains with agendas that advance depending on PC actions/inaction. The PCs can witness the effects of their actions/inaction and set related goals for themselves. The DM can, at the very least, anticipate the pursuit of those goals (and, ideally, the consequent neglect of others).If you know the end, there is a destination, and that means you're on tracks. There may be many paths, but you're still on tracks. Even being flexible means there is still an end goal to get to ... which means you're on tracks.
The idea of a sandbox is that there are no tracks. You can play anywhere in a sandbox and do anything - because you're not trying to get anywhere. You're there to have fun. You just organically play and see what happens. A lot of people think they're running sandboxes because they let the players take path A, B or C to get to a destination. That is still being on tracks. If your players have a goal you expect them to pursue, then you've moved out of the sandbox.
However, there is nothing wrong with having both sandbox and tracks in a campaign. If you watch Critical Role, Mercer is very good at melding both approaches. The Mighty Nein began on a rail - there was a mystery at the circus, and they had to solve it. Then he let them explore and built adventures as they explored. As he did so, he began to pepper in things that related to their backgrounds, things that laid a path ahead for them to follow, etc... In other words, he let them play in the sandbox, but put bright lights around a train station at the edge of the box and lured them to it. They would then get them on a train which they'd follow to complete an adventure, and then enter another sandbox. However, whatever they decided to do, he let them go do - even if it took the train off his tracks.
In no way would I describe such a scenario as anything other than sand-boxy (assuming a sandbox/railroad dichotomy).