Hello!
For now if you build in planning mode trucks consider those plans as built structures and avoid then in pathfinding. I think it's incorrect and ghost buildings should not affect pathfinding. When planning long rail network it is very annoying :)
The risk of this is that making building ghosts "nonexistent" to the pathing system also makes it difficult to control trucks- to keep them from getting stuck in the middle of the build, for example, where they might be hard to click. That said, I have also had issues where I was forced to build some section later because I had to wait for the crossing to build before I blocked the other truck route with another crossing- it might be nice to have a per-ghost, or maybe drag-select, option to say "allow trucks to path through these ghosts". Particularly for traditionally-truck-pathable structures like rail crossings.