It doesn't go away, actually. It stays there until you've completed line placing.
Which reminds me...I have what I think is a bug when using line-placing. I need to replicate it and get a screenshot for Mantis.
Basically, when I'm done line-placing something, the "Line Placing Step 3" text gets tacked on to a bunch of menus, like at the top of the Command Station menu. Not a game breaker, just annoying.