While working on a scenario for the Western Lines of Scotland route, I committed some errors that I'd like to help other scenario writers avoid.
The errors began when I added an AI train scheduled to move westward, pick up a cut of blue TTA wagons located at the far end of the same track, and finally head eastward and exit through a portal. When I tested this, the AI engine crashed into the TTAs instead of coupling to them. End of scenario, of course. This happened because, after placing the AI engine and the wagons and inserting the instructions, I had forgotten to click the "yield sign" icon that warns of errors in the Timetable Editor. You know, the one typically jammed with "consist clash" warnings.
I restarted the editor and, this time, clicked the "yield sign", which reported a couple of duplicate wagon numbers. One of each duplicate made up part of another AI train, while the other occurred in this new AI train. Finding the wagons in my latest AI train and changing their numbers just took a couple of minutes, after which I saved the scenario.
When I attempted to run the scenario again [I always shut down and restart TS2013 completely between saving an editing session and testing the results.], the same crash occurred.
What was wrong now? Still numbers. This time, I had forgotten to reset the wagon numbers in the AI engine's pickup instruction. That took less than a minute; I saved the scenario.
This time, the pick up and pull away actions happened just as they should. It was all a matter of never neglecting the numbers.
P.s.: I've noticed that whoever wrote some of the scenarios that come with the Western Lines of Scotland route avoided such problems by assigning "active" wagons very simple numbers: 1, 2, etc. These numbers look strange when they show up in shunting instructions, but the certainly help avoid the type of errors I've described here.