Multi-track rail routeing

[Julia]

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?

[Carl]

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!

[Ters]

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".

[Roads]

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.

[ӔO]

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.