Signals & Track How To Change Track To Rusty?

So when I drag the rusty material over the redcircled material on the track component it doesn't let me change it to rusty even though I've seen this done in the Peterborough to Doncaster route?

Anyone figured this out?

 

Okay I figured it out,

 

A further update this applies to the UKCP Tracks in the UK Core. I've not tested this with other tracks.