In response to Alexis, here: http://tao-dnd.blogspot.ca/2017/12/everything-is-road.html
This is interesting food for thought, but I don't know that I'm sold on such a broad definition of a railroad - unless I've misunderstood, Alexis is saying any voyage with a destination is now a railroad.
In my mind, the defining features of railroads are the permanence and inflexibility of the route, the scheduled departure and arrival, and the impossibility of changing destination once en route. On a railroad voyage, you are not in control except at stations (where your only choice is to disembark or remain on board until the next station). I cannot board a train and go to my home town. The trains haven't gone there for sixty years. It is impossible. I can go to Montreal, but once I have boarded, I can get off at Kingston or a few other points, but I cannot decide to go to Barrie without disembarking and purchasing a new fare.
Contrast this with a road and a car. I get in, I can pull off the road at any point, stop at a rest stop or just on the shoulder, change destinations at will (along the road network, of course, but at any time). I am in control of the route, the schedule, the destination, and even the moment-to-moment course of the vehicle, whereas with a train I am not.
The word railroad - emphasis on the "rail" - describes the most restrictive form of travel. Even airlines and cruises, which are superficially similar (set schedules and destinations), have more freedom than railroads. The captain can choose to change destinations at any time to anywhere within range with an airport/harbour, say, in the event of a medical or mechanical emergency, or a hijacking.
I don't think it's coincidental that we use the most restrictive kind of travel as a metaphor for the most restrictive kind of games. An rpg railroad doesn't merely have a destination. It is controlled by an engineer. It is running on rails. Your options are to get off (and find another game group), or continue to the destination. You will visit the scheduled adventure sites in the scheduled order. Anything not on the itinerary is just so many telephone poles whipping by at sixty miles an hour.
So, at one end we have literally the most restricted form of travel. At the other, we have the sandbox - the ultimate example of self-directed, structureless play. Even more so than on the road network, I'm in control in the sandbox. I can make a hill and raze it, build a road network and have my toys run along it until I'm bored and I decide it's a river and we're rafting along it, then build a town and a forest and play at bandits.
In a game context, we can farm mustard until we get bored, then wander the roads helping travellers until we tire of that and decide to open a river freight business, or go be bandits in the woods, or anything else.
The distinction isn't between whether there is a direction/destination/goal/series of obstacles, it's who is in charge. In a railroad, the DM is the engineer, the conductor, the train, the rails, the stations, and the tycoon who planned the route, while the player is the Sunday tourist seeing the sights by rail.
In a sandbox, the player is the child and the DM is the sand.
From Alexis' conclusion:
"Choosing to set themselves on the path to find a McGuffin and obtain some clues is not how a sandbox works; it only shows a willingness for players to buy their own tickets for the destination they choose. That's nicer than being told which train we're going to board, but it is STILL a railroad."
I don't know about that. If it's a railroad, then they can't get off, visit the stations in any order, or decide to fuck it and just set up as bandits because it turned out this MacGuffin was boring. If it's a railroad, they'll find the DM-as-conductor putting obstacles in any other path to dissuade them, or sweetening the pot, or otherwise coercing/outright forcing them back onto the rails, because in a railroad, the DM is in charge.
If it's a sandbox, they can decide they don't care about the MacGuffin at any point and make the game about being bandits in the woods, and the DM will roll with it, because the DM is the sand, and there to provide the medium of play.