This Technical Specification defines characteristics and interfaces for electric onboard power supply systems. It applies to locomotive hauled passenger trains and electric multiple units with distributed power as well as trains with concentrated power for main-line application. The objective of this Technical Specification is to define target systems, as regards the following interfaces and characteristics in order to enable further standardisation: •interface between traction system and auxiliary power supply system; •train line type: voltage, frequency and number of poles; •interface between auxiliary power supply system and battery system; •interface of the auxiliary power supply system as well as the low voltage grid to a shore supply (stationary workshop supply or external supply); •supply concepts for essential loads e.g. HVAC systems and battery chargers; •redundancy concept within the supply systems; •auxiliary load control and protection strategy at train level. Described system and interface characteristics define the technical basis for dependent European Standards and Technical Specifications. The introduction of this Technical Specification shows this dependency to adjacent documents. Starting from a generic functional description of electric onboard energy supply systems structured in line with EN 15380 4 and a description of the related vehicle concepts, generic system architectures are derived, which are illustrated by examples of consistent sets of system designs showing interfaces and dependencies among concerned subsystems. Relevant train configuration and concerned energy supply subsystems in scope of this Technical Specification are defined in Clause 4.