What is the solution if AI is running red signals in TS12?

I have never has this happen before in TS12.

Diagnostics, please.

Some illustration or diagram of the area concerned would help to illustrate the questions.

General answer is, AI movement control has never been that great in Trainz and you need to give it as much help as you can. The most obvious thing being correct spacing and sufficient signals to control the approach depending on the track speed. If you have a route with 100 MPH line speed but your signals are only 150 yards apart, even under AI DCC control a train is unlikely to stop in time if it has too short a distance between the caution and stop signal. Trainz also doesn't tend to work very well if you follow strict prototype signalling practice, has its own quirky needs particularly at junctions with multiple track or routing possibility.

You could try popping a speed restriction prior to the signal, say 20 MPH see if that prevents the SPAD.

I found a solution. Use one of JR's signals which one can define a path through the junction block. I set mine to F for farthest.

JR MS Rowen E 04-1,<kuid2:45324:251046:1> by norfolksouthern

AI actually worked smartly when I just tried the above. Held tight on red when another train was controlling the switch on the mainline from the yard and proceeded on green, just like clockwork, once the switch was clear ahead.

The signals on my latest test were also going yellow when they actually SHOULD as well as when the next signal ahead of it is red to allow the train to slow down to a smart speed ahead of time and eventually creep up onto the red signal like clockwork. My signals are at least a half mile apart in a 50 mph zone. Maybe closer in a 35 or 25.

Some of these work well as I have just discovered some of them make AI stupid.

Some of them, AI will actually stop the train but won't be able to change the signal color once the track is clear ahead.

Some of them just get run right through on red.

Another thing, if these signals are stuck on yellow on a main line, make sure there is a number of them along the line and place an invisible signal in between as necessary to make them go green.

AI will act stupid on some of JR's signals but act perfectly if you choose the right kuid signal it happens to like and scatter some invisible signals like breadcrumbs along the route as well.

I had a problem where one specific freight train would coast towards a red signal at 30mph and then slam on the brakes at the last minute, causing repeated SPADs.
The easiest fix I found was to improve the braking performance of the locomotive - I hacked the engine specification and increased the max-decel setting.

If you intend to release the route and session, you will need a different strategy...
I haven't tried it, but maybe you could place a trackmark before the signal and try different driver commands (eg "drive to" "set DCC speed" or "set max speed") to slow the consist down before it reaches the signal?

or the most part, invisible signals are unnecessary when using Jointed Rail or other scripted signals based on the same technology.

I find that by placing Type 05 signals, permissive signals, spaced evenly along the route, the AI will run well even with while following another train. But, i