49 U.S.C. § 20501Section
§ 20501. Definition
In this chapter, "signal system" means a block signal system, an interlocking, automatic train stop, train control, or cab-signal device, or a similar appliance, method, device, or system intended to promote safety in railroad operations.
(Pub. L. 103–272, §1(e), July 5, 1994, 108 Stat. 883.)