Rail Shipper Fairness Act of 2020 This bill revises federal rail transportation law to (1) require rail transportation service to be efficient and reliable based on a shipper's reasonable service requirements, and (2) expand enumerated U.S. transportation policies to provide for and promote the protection of the shipping public. The Government Accountability Office must conduct a study of the impact on freight rail shippers, the National Railroad Passenger Corporation (Amtrak), commuter railroads, railroad employees, and other affected parties of changes in freight railroad operating and scheduling practices as a result of the implementation of the precision scheduled railroading model.