This is fantastic and a feature that I really missed from using Basecamp.
What would take it to 11 out of 10 would be a toggle to automatically show the other routes stored in the same folder as the one that the route that you are editing is stored. This was the fantastic thing about Basecamp. As my routes are stored in folders corresponding to areas, I could highlight a folder that covers the rough area I’m interested in and see all routes in that folder/area.
It can be done using your new functionality, but would currently require manually adding all the routes in the folder to all the other routes in the folder, so would grow exponentially as the number of routes increases.