My economy is taking off and I have a mixture of fast passenger trains and slow goods (limited by the max speed of wagons). I'd like to four-track all my main lines, like this:
->-----A-------\/--------B---------\/--------C----->
->-----D-------/\--------E---------/\--------F----->
<--------------\/-------------------\/--------------<-
<--------------/\-------------------/\--------------<-
and I'm losing track of all the waypoints each time I double up a new stretch (any chance of introducing named waypoints?). In this situation, how reliable is it that trains ordered from A to C without an intervening waypoint will run A-B-C rather than A-E-C, and likewise D-E-F not D-B-F?
It depends. Trains will choose the shortest route from A to C. On straight track like the above, that will lead to trains reliably going A-B-C -- but if this section of track is on a certain kind of curve, A-E-C would sometimes be shorter. In practice you'll only need to place waypoints at the places where trains would otherwise route themselves wrongly. This may take some trial and error!
Trains don't choose what is strictly speaking the shortest route. Straightness, both horizontally and vertically, is also taken into account. I recently had to manually route some trains onto a less congested shortcut that was deemed slightly more "costly" than the somewhat longer route due to a few turns, two bridges and a hill. I only needed to do this for one direction, as the track in the other direction didn't need the two bridges and was therefore "cheaper".
The thing that convinced me to move to version 111.3 was the updated graphic for location/way points. I love that red square! I use a naming convention to keep up with which line has which way points.
I'd agree. Try the latest official release, which is 111.3.1.
You can also try it like this:
<---< Passenger
>---> Passenger
<---< Freight
>---< Freight
In which case it should be easier to move one of the services off to the other track.