Did you try a track mark after the switch?
AI driver sees the signal and switch, comes to the switch and then loses sight of the path to the portal. Have the driver set its sights on the track mark after the switch then have the AI driver continue on. If you have other switches, have track marks after those.
It doesn't have to be track marks if you have passenger stations, for example, or some industry the AI has to drive to, but in essence they are one in the same in this case.
John
Trainz User Since: 12-2003
Trainz User ID: 124863
TRS2019/Trainz-PLUS: 114800
TRS22 Beta tester