Steam Stop Trains From Leaving Slow Lines In Timetable

Hi Guys.
I am trying to put 2 Class 86's with a Container Train on the WCML in a Timetable route, but, the routing keeps crossing the train over onto the fast lines.
Can someone please tell me how I can stop that from happening when I set the Destination>
Any help would very much appreciated.
Thanks Guys.

 

In the map, click on the train (after having set your destination), then select Add Waypoint (I think?, Go Via?), then select the slow line further up the map.
The 86s aren't configured to work in multiple, it's worth noting

 

Hi Strat-tastic.
Thankyou for your reply, it was my error why I couldn't change the route, all good now though.