Read this for CRN versions from beta-0.8.0+

Train groups are only used to filter the search results when navigating and have no other functionality. Train groups are created in the Global Settings, but must be assigned to trains in a New Schedule Section entry in the train schedule. This means that a train can change its train group during its journey if desired. You can also select a color, which is then used for the trains in the navigator. If no color ist selected, the default dark gray color is used.

[!NOTE] If a train is assigned to a Train Line that also has a color, this color will be displayed instead of the Train Group color.