Lady McAlpine has asked auction houses, dealers and collectors to be aware if they are offered signs for sale.
The railway was started by Lady McAlpine’s late husband Bill McAlpine in the 1960s when he was working for his family’s construction firm, Sir Robert McAlpine (founded by Bill’s great-grandfather in 1869).
The railway network includes a restored Victorian station, known as Somersham Station, the steepest standard gauge railway track in the world and a museum. The venue is sometimes open to the public for organised events during the year.
Anyone with information should call the police on 101 quoting the crime reference number 43200006334.