Unsupported elevated rail collapse even in planning mode

As it says on the tin, if you plan elevated rails without sufficient support, they will collapse in a few seconds.

My expectation would be that I can create 'impossible' situations while planning
.

sounds almost intentional - it's better to know something will collapse before you designed everything around it not collapsing

But it already has the 'building in danger of collapse' icon. I don't think it needs to collapse in planning mode