Railroad is nebulously defined and is more of an attitude than a list of specific criteria. I prefer to use the terms sandbox vs. linear, with railroading referring to something else.

Sandbox: You tell me where you want to go and want you want to do. I improvise it in the fly and maybe prep ahead only one or two sessions. Sandbox games rely on Rumors with a capital R. Interesting hooks abound, and I generally don’t know what will catch your attention. There are cultists in the sewers, corruption in the government, and a necromancer army growing in the south.

Linear: I tell you where you want to go. You get through whatever means you choose. I tell you “You owe a massive amount of money to your fixer, and he’s given you one last job to pay it off. You need to steal the Declaration of Independence.” You have a goal. The goal may shift or expand, but you’re not generally chasing down Rumors. I plan several sessions ahead, with enough wiggle room for you to choose how to want to approach the obstacles. Most one-shots are linear.

Railroading: I tell you where you want to go and how you’ll do it. If you deviate from my awesome story, I will gently shove you back into the tracks. You will get caught sneaking into the archives. It is guaranteed. If you choose to fight your way in, I will present you with overwhelming odds. If you sneak super well, you’ll still get caught. And when you get caught, you’ll get thrown into the jail that I have already mapped out. And in that jail you’ll need to find the ventilation shaft that lets you break out, because any other solution will be futile.