What version of Trainz are you using? Trainz Plus has a HD option - have you converted these routes to HD?
From the image it appears to be a spline height issue and not a texture issue. So my first solution would be to use the Smooth Ground Under Selected option in the spline context menu (Surveyor 2.0). There is a similar command somewhere in Surveyor Classic but it has been so long since I used that interface.
It can also be the 'shadow quality' setting in launcher/performance. Any higher than 'medium' and all the gaps under rails will show black throughout the route.
Or, possibly nearby PBR. As a simple test change the nearby grass to a TS19 type such as FR9 (on DLS). I have grown tired of all the black streaks and chasing each one. So I switched to FR9 grass totally. Other nearby PBR elements will put their "bubbles" under the track as well.. Thus a nearby PBR asphalt will cause the streaks.
My guess is that it is either an issue that has to do with pbr, or is just a bug with the game that hasn't been fixed. You may have to go through and fix it manually.
@WCL >>> I see a second roadbed spline under the trackbed spline. You likely just need to edit the spline points in the lower bed spline, or, add a section of that lower bed spline in parallel to fill in the gap