The CANopen application profile for train vehicle control systems (CiA 421) describes the communication within the integration network that facilitates information exchange between sub-systems located in a single rail vehicle or a consist of rail vehicles. It provides a detailed description of the application data communicated by rail vehicle sub-systems, such as the door control system, passenger information system, HVAC, and others.

For instance, when it is necessary to close all doors of a rail vehicle, the train operating system needs to know how to send the appropriate command to the door control system. From a CANopen perspective, this involves determining which object in the CANopen object dictionary of the door controller should receive the "close all doors" command. Additionally, the door controller has to understand the structure of the command (e.g., which bit specifies the requested action). This and other related information are standardized in the CiA 421 document.

A rail vehicle integration network is typically connected to a "train bus", which is the embedded control network enabling communication between rail vehicles. Traditionally, a wired train bus (WTB) is used. CiA 421 includes provisions for interfacing with a WTB-based train control network via a gateway device. To reduce the formatting workload within such a gateway, the data defined in CiA 421 has been harmonized with that specified in UIC Leaflet 556.

The CiA 421 series of documents defines interfaces for virtual sub-systems used in locomotives, power cars, or coaches. CiA 421-based applications form a part of the virtual multiple network architecture. They represent a higher-level hierarchical virtual network at the rail vehicle level, integrating multiple sub-systems. The CANopen application profile CiA 421 defines a significant amount of harmonized data to simplify system integration. Unlike many other CANopen application profiles, CiA 421 avoids specifying pre-defined PDOs. It provides communication interfaces for the following sub-systems:
 

• Part 1: General definitions
• Part 2: Train operating system
• Part 3: Monitoring and safety system
• Part 4: Auxiliary operating system
• Part 5: Power (drive) system
• Part 7: Brake control system
• Part 8: Ancillary operating system
• Part 9: Vehicle linkage system
• Part 10: Exterior lighting system
• Part 11: Interior lighting system
• Part 12: Door control system
• Part 13: HVAC system
• Part 18: Slip/slide control system