The Alexandrian " The Railroading Manifesto - Part 2: Methods of the Railroad
Railroading can take a myriad of forms, but it tends to boil down into a few major methods. ENFORCING FAILURE: The PCs cast a teleport spell to bypass the frost giants guarding the Necromancer's inner sanctum (that's the player choice), but you want them to meet the Frost Giant King (that's the preconceived outcome), so you decide to add a teleport interdiction field to the inner sanctum (that's enforcing failure).
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The Railroading Manifesto - Part 2: The Methods of the Railroad