49 CFR §174.85

1. (a)Except as provided in paragraphs (b) and (c) of this section, the position in a train of each loaded placarded car, transport vehicle, freight container, and bulk packaging must conform to the provisions of this section.
2. (b)A car placarded “RADIOACTIVE” must comply with train positioning requirements of paragraph (d) of this section and must be separated from a locomotive, occupied caboose, or carload of undeveloped film by at least one non-placarded car.
3. (c)A tank car containing the residue of a hazardous material must be separated from a locomotive or occupied caboose by at least one rail car other than a placarded tank car.
4. (d)Position of rail cars in a train. In the following table:
   1. (1)Where an “X” appears at the intersection of a Placard Group column and a Restriction row, the corresponding restriction applies.
   2. (2)“Rail Car” means a car other than a tank car.
   3. (3)For purposes of this subpart, each unit of an articulated intermodal rail car shall be considered as one car.