I'm trying to render a line feature service with a cased symbol. For those of you unaware, a cased symbol is essentially like drawing an outline around the line. It creates a very nice visualization. My current workaround is to create a copy of the feature service and draw the copied layer slightly larger. This creates the effect but now I have two layers in my contents pane and this is undesirable. I'm surprised by ESRI's shortcomings in regards to the rendering options they provide in ArcGIS Online. From what I've seen it looks like they provide very few options.
ArcGIS Online is underdeveloped in general. I think your best option would be adding a small buffer layer around your polylines and choose whatever colour is most visually appealing for you.