As an alternative to set waypoints in the Line's schedule as a way to force a convoy to pick a certain route/track/road it might be a good idea to add variable weights to tiles.
As I understand a convoy take the shortest distance when it looks for the route to take to the target. If I then can change a piece of track or road to look as 10 km instead of 1 km I can trick my convoy to not take the shortest route.
When using waypoints you "force" vehicles to go there.
When using variable weights on tracks/roads you can instead force unwanted vehicles to avoid that piece of track/road.
You can find the same thinking when you set up routes in internet protocol routers.
best regards
Anders
Actually the maximum speed is taken into account. The problem is that curves have quite an impact and as such the alternative strech must be considerably faster. A high speed train will use faster side track over 120kmh normal track when they are longer than roughly 50 tiles.