This document defines the interfaces and gives guidance for the design of systems and procedures for change of track gauge. It defines also their assessment for technical approval, for the automatic variable-gauge systems. The document is focused on the change of track gauge among the following nominal track gauges: 1 435 mm, 1 520 mm, 1 524 mm, 1 600 mm and 1 668 mm. This document is not limited to the aforementioned nominal track gauges but the interfaces to change to/from other nominal track gauges can be different. The established assessment procedures can be used as well.