The Hidden Staircase Nancy Drew Book by Carolyn Keene
Starts out with Nancy helping her dad Carson with railroad business. He has to find a man that signed off on the original railroad property. He has to leave town to do that as he can't locate the man locally.
She gets a visit from the owners of the mansion and it's twin house asking her to help them solve the mystery of the ghost in their house. Strange things have been happening and they want to know what to do about them. This book takes many turns, a kidnapping and secret passage ways among the two houses.
Loved the exploring and how she accumulated the clues to actually solve both mysteries.