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Experienced certain patches where landfill was laid down to cover water and later track could not be laid over those sections. This is a water-heavy map and we have laid multiple land bridges that were able to support train rails running out to several islands. I encountered this error when using landfill near where coastal borders had been, specifically with train track because I was most likely to use a few pieces of landfill to flesh out a corner that would otherwise have been impossible due to the water, however, it was not a consistent error over coastal landfill.

If you pick up landfill, extend its size using Numpad+ and hover over the area - maybe you will find 1 tile lakes which is just not displayed as such but is still considered a water tile? Landfill will display it as placable (greenish) tile

Another guess - entities can't be placed right near the water. I am not sure if it checks for water tiles or ground tiles are flagged as unplacable during map generation. And if you do landfill they might not be fixed? This might be a bug but I truly doubt this is the way game works as it sounds pretty silly to me compared to checking for current water tiles.

Thanks for the report.
In the 3rd image the curve is too close to the lower corner. That's how it is now with land-water transitions drawn on water tile instead of land tile.

In the other cases you can't build because colliding with forgotten single water tile, which is almost perfectly hidden under transition of two terrains meeting. We will work on making those more visible: 54663

Looking at this problem I would suggest devs to not only tint placed entity red but also highlight conflicting tiles to identify collision point.