; and (6) the use of hotbox detectors to prevent derailments from wheel bearing failures, including— (A) the temperatures, to be specified by the Secretary, at which an alert from a hotbox detector is triggered to warn of a potential wheel bearing failure; and (B) any actions that shall be taken by a rail carrier upon receiving an alert from a hotbox detector of a potential wheel bearing failure. (c) Defect and failure identification The Secretary shall specify the categories of defects and failures that wayside defect detectors covered by regulations issued pursuant to subsection (a) shall address, including— (1) axles; (2) wheel bearings; (3) brakes; (4) signals; (5) wheel impacts; and (6) other defects or failures specified by the Secretary. 6. Safe Freight Act of 2023 (a) Short title This section may be cited as the Safe Freight Act of 2023 . (b) Freight train crew size Subchapter II of chapter 201 of title 49, United States Code, is amended by inserting after section 20153 the following: 20154. Freight train crew size safety standards (a) Minimum crew size No freight train may be operated without a 2-person crew consisting of at least 1 appropriately qualified and certified conductor and 1 appropriately qualified and certified locomotive engineer. (b) Exceptions Except as provided in subsection (c), the requirement under subsection (a) shall not apply with respect to— (1) train operations on track that is not a main line track; (2) a freight train operated— (A) by a railroad carrier that has fewer than 400,000 total employee work hours annually and less than $40,000,000 annual revenue (adjusted for inflation, as calculated by the Surface Transportation Board Railroad Inflation- Adjusted Index and Deflator Factor Table); (B) at a speed of not more than 25 miles per hour; and (C) on a track with an average track grade of less than 2 percent for any segment of track that is at least 2 continuous miles