After an hour of experimenting with different trackmark locations I think there are two things working against me to keep Ben Neal's 2-8-2H from overshooting the platform. A combination of Ben's loco and the pencil42's platform. I was able to get other locos to stop at the platform if used with an additional trackmark that was placed just at the right distance. Experimenting more with Ben's 2-8-2H it's challenging to keep it from overshooting a regular trackmark at any speed over 20mph.
Just for reference, why doesn't changing the platform trigger radius values in the config to 15, 25, or even 50 affect the behavior of any AI driven locos?