TimelineCollection defines no methods or properties beyond the basic Collection methods and properties. Methods such as Add or GetItem will expect or return objects that are of type Timeline.
The XAML syntax for properties that use a TimelineCollection is an example of implicit collection syntax, in which you omit an actual TimelineCollection object element. Instead, you generally include one or more animation types (Timeline elements) as child elements of a Storyboard. In this case, the explicit syntax is not permitted; the parser accepts neither a <Storyboard.Children> property element nor a <TimelineCollection> object element.