Start the animation with Path is loaded


   
      

<Canvas xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Path Canvas.Left="150" Canvas.Top="150" StrokeThickness="25"
          Data="M 0 -100 
                C -55 -100, -100  -55, -100 0
                S  55 -100,    0 -100">
        <Path.Stroke>
            <LinearGradientBrush SpreadMethod="Repeat">
                <LinearGradientBrush.Transform>
                    <TranslateTransform x:Name="xform" /> 
                </LinearGradientBrush.Transform>
                <LinearGradientBrush.GradientStops>
                    <GradientStop Offset="0.00" Color="Red" />
                    <GradientStop Offset="0.14" Color="Orange" />
                    <GradientStop Offset="1.00" Color="Red" />
                </LinearGradientBrush.GradientStops>
            </LinearGradientBrush>
        </Path.Stroke>
        <Path.Triggers>
            <EventTrigger RoutedEvent="Path.Loaded">
                <BeginStoryboard>
                    <Storyboard TargetName="xform" TargetProperty="X">
                        <DoubleAnimation From="0" To="621" 
                                         Duration="0:0:2"
                                         RepeatBehavior="Forever" />
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
        </Path.Triggers>
    </Path>
</Canvas>