Add overlaying line to map that can move together with the map

[Yqy]

i have a railway map in flow map, i need to seperate it into different areas by having overlaying transparent lines. The overlaying lines must follow the map as i zoom in & out or move the map around. If not possible to do so in flow map can suggest another visual or method?

For example:

Hope someone can help me to solve this problm, thanks.