((a)) ** .—**

((1)) When the Secretary of Transportation decides after an investigation that it is necessary in the public interest, the Secretary may order a railroad carrier to install, on any part of its railroad line, a signal system that complies with requirements of the Secretary. The order must allow the carrier a reasonable time to complete the installation. A carrier may discontinue or materially alter a signal system required under this paragraph only with the approval of the Secretary.

((2)) A railroad carrier ordered under paragraph (1) of this subsection to install a signal system on one part of its railroad line may not be held negligent for not installing the system on any part of its line that was not included in the order. If an accident or incident occurs on a part of the line on which the signal system was not required to be installed and was not installed, the use of the system on another part of the line may not be considered in a civil action brought because of the accident or incident.

((b)) ** .—** A railroad carrier may allow a signal system to be used on its railroad line only when the system, including its controlling and operating appurtenances—

((1)) may be operated safely without unnecessary risk of personal injury; and

((2)) has been inspected and can meet any test prescribed under this chapter.