It is there by default if you are holding a rail signal I think.  

...but you need to be holding a rail signal near the rails for it to appear. Great feature IMO.

https://www.reddit.com/r/factorio/comments/7kg0o4/woah_whats_this_and_how_do_i_use_it/?st=JBDI9C2E&sh=9255de26

Already on by default 

https://www.reddit.com/r/factorio/comments/7kg0o4/woah_whats_this_and_how_do_i_use_it/?st=JBDI9C2E&amp;sh=9255de26

Already on by default 

In 0.15 I used to *live by* the rail segments visualization - `show-rail-segments` in the F4 debug menu. Now that I've switched to 0.16 (.6 at the moment) it looks like it has been removed. If anything, I thought it was going to get *better* (source: [FF 198](https://www.factorio.com/blog/post/fff-198)).

Have I missed an option somewhere?

How to get rail segments in 0.16?