49 C.F.R. § 238.137
Mixed consist; operating equipment with incompatible exterior side door systems
(a) A train made up of equipment with incompatible exterior side door systems shall be operated within the constraints of each such door system.
(b) No later than December 6, 2018, each railroad shall adopt and comply with operating rules to provide for the safe use of equipment with incompatible exterior side door systems when utilized in a mixed consist.