I finally got back to this one. Here's what I did, you can adapt it to your requirements. BTW this is in TRS2004 but all later versions should be identical or very similar.
Layout:
One baseboard, one straight track.
In order:
Where the loco starts
Trigger TR1
Trackmark TM1
Trackmark TM2
Trackmark TM3
(It helps to put a building alongside each trigger/trackmark so you know where they are when driving)
Place a loco at the starting position
The objective is to make the loco drive to TM1, then drive back and forth between TM2 and TM3 forever.
Rules (where each > represents one level of indent):
Startup options - as normal
Driver Setup - Drive to TM1
Driver Command - as normal
Trigger Rule - Trigger TR1
> Ordered List - This process will be repeated 1 time
> > Driver Setup - Drive to TM2, Drive to TM3, Repeat
I hope you can copy this and get it going, and that it's of some help to you. I find one of the trickiest things is to keep the difference between Rules and Driver Commands clear in my head.
John