33 minutes ago, Taki said:the point of question comes from the section on collision where it mentions terrain you didn't start the turn in contact with. That creates the question, it's not cut and dry.
55.3 "If a unit would overlap an obstacle while moving, that unit’s movement is halted. Then, the unit slides backward along the movement template until it is touching the obstacle, but not overlapping it. The unit collides with that obstacle."
If you start next to an obstacle, then as you move forward, there will be many points at which you overlap the obstacle, so the movement is halted.
The part you are referencing is 18 here:
"After a unit performs a march (?) or shift (?) action, if it is touching an obstacle that it was not touching before performing that action, it has collided with that obstacle."
The only time this really comes into play is when you shift to re-position while engaged to an enemy. That makes it so you don't collide and therefore don't take another panic token.
So it appears that collision and overlap are different here. Certain affects occur on collision. You can receive panic tokens (if you didn't charge) upon collision with an enemy. According to 88.1:
"If a piece of terrain has a capacity value, a unit can choose to enter and occupy a piece of that terrain when that unit collides with it."
So you can enter terrain upon collision though apparently not if you were already touching it. That bit does seem a bit silly and I doubt that they intend it that way (because otherwise you wouldn't be able to enter terrain after destroying the enemy unit that occupied it since you touched it before). I'm sure the spirit of this is that you can enter it when you move and the other requirements are met.
I may be slightly wrong on some things here (I'm not aware of anything though), but in general it seems clear to me that you can't pass through terrain.
EDIT: See I read so many things I confused myself. As you see in 55.3 it says at the end that the unit collides with that obstacle so it is a new collision and you do have a new chance to enter the terrain. Reposition wouldn't fall under this though so you obviously still don't take the panic token.
So yeah, TL;DR you definitely don't pass through the terrain unfettered. You collide again and have a new chance to occupy.