I had a problem like that way back in TANE. Details are hazy, but I'm sure I cloned a track and gave it the KUID of the missing track, called it MissingTrack, and then drag/dropped it back into CM. This way, once you re-entered the route,the white lines are gone (replaced by the other track), and then you could use the Replace Track function to change the current track to something you have. I'm pretty sure I used Auran's Wooden Toy Track, which made the track very apparent on your route.
Bill
That's one-way to do it.
Here's another I got this from James Moody way back in TS12 days.
The dashed white lines isn't "a missing asset" - it's the asset that Trainz turns to when it can't find the asset it is supposed to draw there. These white lines are, in themselves, an asset. You can find this asset in CM. It's called "Missing spline", and it's by Rob.
Open it for edit.
Change the KUID from <kuid:523:1124> to <kuid2:523:1124:1>, and remove the whole privileges container (containing the line "permit-listing 0")
Then install that asset as if it were a new asset - e.g. drag the folder from windows explorer back onto the CM interface.
Once it's installed, revert the original one (you can't commit changes to it as it's a built-in asset).
Result: The dashed white lines now show up in the menu, and can be selected just like any other track spline. You could lay some down if you wanted. More to the point, you can now use the 'replace items' tool to change them to any track spline you have installed...
I will add this. When you install the folder, use the install content option. This will save so much time. Also don't panic when the creator is shown as AURAN. I hope this helps someone.
Notes:
<kuid2:523:1124:1> ==== Update from <kuid2:523:1124:2> to <kuid2:523:1124:3> for T:ANE and up unless the assets have been updated since. All other procedures apply.