When I first started running AI driven trains I was downright amazed at how AI drivers would want to 'shortcut' through a maze of rusty industrial sidings with a speed limit of 5 mph instead of taking the open and clear mainline route right in front of them. Being a UK pre-grouping modeller facing crossovers are rare on my lines so I don't have too many problems with the AI idiots wanting to switch tracks for no reason at all, but I still need to sow 'go-this-way-you-idiot' trackmarks at the major junction stations if I want things to run smoothly.
Very well said. This has been my experience as well. It really frosted my cookies the first time I saw my crack express passenger train head down the crooked and rusty branch instead of staying on the mainline. The issue is the driver "sees" the bad track as the shortest way between two points due to the junctions even if they don't appear that way to us.
I use Drive via and Navigate via track mark commands in those places where there are going to be issues. Placing a track mark in the beginning, just as a section begins, in the middle, and then at the end really helps. Using the Drive via, and not the Navigate via, they AI will tend to wait and the drive command is more specific. Navigate will mean they will find the shortest way between two points and do stupid things, but the Drive version means do this with no faffing around!
I do the same with wye-type junctions (triangle-junctions). Without direction the AI, the driver will take the wrong leg of the wye and end up doing a back up move that's beyond explanation. To avoid this idiot move, I place 6 track-marks. One before and after each leg on the mainline side plus one on the middle, and then one on each branch, plus one at the very end on the branch its self.
I then direct the AI to drive via track marks through the junction and they don't do the backup, reverse, get stuck, turn around, get stuck again, and then proceed after I untangle them move.
Where there are crossovers, I will either place direction markers as one-way signs to prevent the AI from randomly crossing over, and in these areas only I do the manual moves. In other places I will use the direction markers and sometimes track marks to keep the AI on their respective routes if I need them to take the crossovers to a branch or siding.
After doing this for about 16 years, I now anticipate what the AI might do in a particular area. This experience helps with setting up driver command-schedules.