Here's a procedure found in the old tech blogs, which I saved to my notes for future reference. It does work, or it did back when I used this method in TS12.
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...