49 C.F.R. § 236.1027 – PTC system exclusions.

(a) The requirements of this subpart apply to each office automation system that performs safety-critical functions within, or affects the safety performance of, the PTC system. For purposes of this section, “office automation system” means any centralized or distributed computer-based system that directly or indirectly controls the active movement of trains in a rail network.
(b) Changes or modifications to PTC systems otherwise excluded from the requirements of this subpart by this section do not exclude those PTC systems from the requirements of this subpart if the changes or modifications result in a degradation of safety or a material decrease in safety-critical functionality.
(c) Primary train control systems cannot be integrated with locomotive electronic systems unless the complete integrated systems:
1. (1) Have been shown to be designed on fail-safe principles;
2. (2) Have demonstrated to operate in a fail-safe mode;
3. (3) Have a manual fail-safe fallback and override to allow the locomotive to be brought to a safe stop in the event of any loss of electronic control; and
4. (4) Are included in the approved and applicable PTCDP and PTCSP.
(d) PTC systems excluded by this section from the requirements of this subpart remain subject to subparts A through H of this part as applicable.