I suspect arcade doesn't handle things like rotation according to what is show in this guide.
https://developers.arcgis.com/arcade/function-reference/
but arcade is used in things for symbology etc
http://pro.arcgis.com/en/pro-app/help/mapping/layer-properties/vary-symbology-by-rotation.htm
What geometries are you using to label your routes? If they are lines, your labels would follow the line geometry. If they are points, you possibly don't have the rotation information. What software are you using? ArcGIS Pro or ArcGIS Online? Options depend on that too.