Like in old Operation Flashpoint, aka Arm 3 today. The AI soldiers all marched to a single corner in a game scenario my brother and I created causing the game to hang and crash. We couldn't figure out why everything became a slide show until we found them all stuck in a corner. This only happened once but it goes to show what AI can do.The real issue is that AI in every single game is trash. I've yet to play one game in my 47 years of gaming where AI does what I want it to do. AI is only as smart as the person that codes the script in the first place. I used to space my signals every 4 to 7 miles unless there was a junction or some other oddity. Now I usually go a mile to a mile and a half for signal spacing. So when I send an AI driver to point X or Y, it just works. One way markers are useless because a lot of folks use multiple mainline tracks, and only in a rare location are they useful. Priority is something I tried and it works fine, but it isn't perfect. KISS is a thing for a reason (and I'm not referencing the band).
In the 80s we had "One to Grow On" and "The More You Know" during Saturday morning shows and cartoons. Sound advice.
Back to Trainz...
I tend to keep my signals close together because I run mostly short fast commuter trains. With my short blocks, I've never had many issues with the AI drivers except stopping at greens over time. More recently, I've had them stop at a station and not proceed. Placing a drive via track mark <name> in between these particular stations and that solved that problem.
Speaking of track marks and direction markers. Direction markers serve as one-way signs and can be used to prevent the AI drivers from using sidings. They also work to force right-hand driving because by default the AI use left-hand driving because the developers used the Australian/UK driving rules by default.
Track marks are helpful as waypoints to guide the AI through complex track and to prevent them from taking short-cuts through yards. I use them at wyes to ensure they drive through and don't perform screwy back up moves as I've witnessed on some routes.
Some route builders do go overboard with track marks. I have found having too many of them can cause performance issues due to the AI hunting and seeking routes and checking each and every track mark for the correct route.