I was on a EN World hiatus so I missed the early part of this discussion. All i want to say is this:
I prefer the term "rollercoaster" to "railroad" simply because "rollercoaster" reminds us that these sorts of experiences can be fun, thrilling even. As long as everyone buys in, linearity and agency are not necessarily an impediment to fun. I do think that how "railroady" a adventure or campaign is going to be should be presented upfront (the first form of player agency is buy in, after all) but once that is done, go wild.
BG: Avernus is a great example of a super fun railroad aka rollercoaster. You CAN run it as an open world, but the module itself is written as a railroad. Its flowcharts are unbranched columns, for goodness sake.
This is a good point, and I think "rollercoaster" is a good word for the good kind of experience that isn't a railroad, but is cut from the same cloth, just as I had previously articulated "wasteland" as the
bad kind of experience that isn't a sandbox, but is cut from the same cloth.
Both words emphasize common pitfalls that can result from the respective style. A rollercoaster that isn't exciting is...just a weird train ride with no stops. A coaster that isn't
being driven forward is just a carriage sitting there doing nothing, waiting for the next thing to happen. A coaster needs to have thrills and frights and moments to catch your breath, otherwise it gets too sedate and you start wandering away from the experience mentally, which draws your attention to how you're trapped in a chair forced to follow a line. Etc.
Meanwhile, a wasteland is empty, where a sandbox should be, if not "full" in the strict sense, at least well-stocked. A wasteland is dry, where a sandbox has varying elements (wet sand is a lot easier to build sandcastles out of, after all). A wasteland is big and open, but wandering around doesn't
achieve anything, nor does it really have any meaningful impact. You need cool things worth witnessing, events that unfold, forces in contest with one another--and adventures just over the horizon that are always
worth seeking out, even if they don't always end up being the coolest thing ever.
A rollercoaster is the highest height a linear game can aspire to--and when done well, it's a great romp. A sandbox is the highest height an open game can aspire to--and when done well, it's a great romp. But when they devolve into railroads and wastelands, nobody really benefits, and most people would probably be upset to deal with such an experience.