49 CFR §174.201

1. (a)Except as provided in paragraphs (b) and (c) of this section, cylinders containing Class 2 (gases) materials being transported in a rail car must be:
   1. (1)Securely lashed in an upright position so as to prevent their overturning;
   2. (2)Loaded into racks securely attached to the car;
   3. (3)Packed in boxes or crates of such dimensions as to prevent their overturning; or
   4. (4)Loaded in a horizontal position.
2. (b)Specification DOT-4L (§ 178.57 of this subchapter) cylinders being transported in a rail car must be loaded in an upright position and be securely braced.
3. (c)Cylinders containing Class 2 (gases) materials may be transported in stock cars, gondola cars and flat cars. However, they may not be transported in hopper bottom cars.