Question: I own all four railroads, but one is mortgaged. How much rent do I collect?
Answer: You collect the full rent, $200 in the standard game, when someone lands on one of the railroads you own that is not mortgaged. Of course, you collect $0 if they land on the mortgaged one.
The fact that one railroad (or one property in a color group) is mortgaged does not affect the rent owed when players land on other railroads (or other properties in that same color group).
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