The Hidden Staircase by Carolyn Keene

After receiving a call from her friend Helen Corning, Nancy agrees to help solve a baffling mystery. Helen's Aunt Rosemary has been living with her mother at the old family mansion, and they have noticed many strange things. They have heard music, thumps, and creaking noises at night, and seen eerie shadows on the walls. Could the house be haunted?

Just as soon as she hangs up the phone, a strange man visits Nancy's house to warn her and her father that they are in danger because of a case he is working on buying property for a railroad company. This warning leads Nancy and  her father Carson to search for the missing Willie Wharton, a landowner, who can prove he signed away his land to the railroad and save the railroad from a lawsuit. Will Nancy be able to find the missing landowner and discover how these mysteries are related?