Just loaded up Trainz! To best explain all that confusing stuff LOL
Probably best to use Surveyor 'original'.
1) Open the tracks tools menu, then into Trackmark
2) Place down a red track marker 'The red one'!
3) Notice that by default the distance value is set to 20 meters, you can change this while you are still in Trackmark mode
to new track markers or already placed markers
4) Change it's direction facing 'if you like' doesn't really matter much unless its a directional marker. Then Expan the 'Advanced' menu 'clicking on that very small arrow'
5) Now change the value from 20.00 to 0.005
6) Click the button 'set value radious' next to the value of 0.005 meters box
7) Click on the red track marker that you have placed or want to change it's distance value from 20 meters to 0.005 meters..
In the editor track markers will read 0.00 meters.. But it's intentional!

Hopefully that helps mate
Edit! Now set you AI to speed through

You will notice a very, very slight slow down, which can't be helped! But it's a much better setup.