49 C.F.R. § 299.421 – Emergency roof access.

(a) Number and dimensions. Each passenger car shall have a minimum of two emergency roof access locations, each providing a minimum opening of 660 mm (26 inches) longitudinally (i.e., parallel to the longitudinal axis of the car) by 610 mm (24 inches) laterally.
(b) Means of access. Emergency roof access shall be provided by means of a conspicuously marked structural weak point in the roof for access by properly equipped emergency response personnel.
(c) Location. Emergency roof access locations shall be situated so that when a car is on its side—
1. (1) One emergency access location is situated as close as practicable within each half of the roof as divided top from bottom; and
2. (2) One emergency access location is situated as close as practicable within each half of the roof as divided left from right. (See Figure 1 to this paragraph.)
(d) Obstructions. The ceiling space below each emergency roof access location shall be free from wire, cabling, conduit, and piping. This space shall also be free of any rigid secondary structure (e.g., a diffuser or diffuser support, lighting back fixture, mounted PA equipment, or luggage rack) where practicable. It shall be permissible to cut through interior panels, liners, or other non-rigid secondary structures after making the cutout hole in the roof, provided any such additional cutting necessary to access the interior of the vehicle permits a minimum opening of the dimensions specified in paragraph (a) of this section to be maintained.
(e) Marking instructions. Each emergency roof access location shall be conspicuously marked with retroreflective material of contrasting color meeting the minimum requirements specified in § 299.423. Legible and understandable instructions shall be posted at or near each such location.