Railroads are a unique type of Property that increases in value depending on how many the player own, opposed to their own unique upgrades. Until a player owns all four, the Railroads can never be “Locked” or “Upgraded” and are always considered to be Properties that can be Stolen.
Once a player eventually steals or buys their fourth railroad, then all four railroads are considered Locked for that player. At this point, no other player can claim any of the railroads. Railroads Do Not award a bonus.