; (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; (3) locomotives performing assistance to a train that has incurred mechanical failure or lacks the power to traverse difficult terrain, including traveling to or from the location where assistance is provided; (4) locomotives that— (A) are not attached to any equipment or are attached only to a caboose; and (B) do not travel further than 30 miles from the point of origin of such locomotive; and (5) train operations staffed with fewer than a 2-person crew at least 1 year before the date of enactment of this section, if the Secretary determines that such operations achieve an equivalent level of safety as would result from compliance with the requirement under subsection (a). (c) Trains ineligible for exception The exceptions under subsection (b) may not be applied to— (1) a train transporting 1 or more loaded cars carrying material toxic by inhalation (as defined in section 171.8 of title 49, Code of Federal Regulations); (2) a train transporting— (A) 20 or more loaded tank cars of a Class 2 material or a Class 3 flammable liquid in a continuous block; or (B) 35 or more loaded tank cars of a Class 2 material or a Class 3 flammable liquid throughout the train consist