The need to set up an onboard railway network arose some years ago, at first for distributed diagnostic information to the driver. The use of electronic devices on board, since the end of the seventies, has necessitated train drivers and maintenance staff to be able to quickly obtain the status of the whole control system and to send condensed messages and instructions in case of faults. The requirement of more powerful onboard communication systems arose in the beginning of the eighties, when trains were equipped with more sophisticated devices and the need for data exchange grew very quickly. This book deals with the current communication technologies on board trains and describes the current trends in Industrial Ethernet. Then, it briefly presents some technologies used in other embedded industries, especially the automotive. It analyses the railway needs and the development of these needs. Finally, it evaluates Ethernet-based technologies for railway purposes. This work follows up the previous book "Les réseaux de terrain embarqués dans les transports guidés" (Les collections de l'INRETS, 2004).