These are generic missing spline assets used to mark track and other splines when they are missing. They can represent grass, roads, fences, power lines, and track.
In this case, you clearly have missing track. What you need to do is go into Content Manager and find out the KUID of your missing dependency then download that from wherever it comes from. Being a generic replacement, you can't just replace the spline with track because if you have track, a grass spline, as well as a fence too you, you'll have a mess figuring out things.
So...
In Content Manager, right-click on the route.
Pick View dependencies.
A new Content Manager window will open up.
Click on status and sort so that the missing asset(s) appear at the top of the screen. If Status does not show, right-click on the columns and add it, and position it wherever you like. By default it will go to the end.
With the Kuid in hand, you can now search for the asset. It could very well be available from Jointed Rail, TPR or one of the other more common website, or perhaps it's not. There's no way of knowing until you search.
If you do need to download this from Jointed Rail or TPR, you need to sign up, and both places are free.
There is a nice, pay-for-use, website which will assist you with your search. Trainz Kuid Index,
www.trainzkuidindex.com is worth the investment for this.
Jointed Rail website is:
www.jointedrail.com
TPR - aka Trainz Pro Routes:
http://www.trainzproroutes.org/
There are many, many others.