I agree with pware. If you tell us the name and KUID of the asset, we most likely can direct you to where it may be located whether that be DLS or elsewhere. In the event that it's totally missing there is the procedure outlined below. This might be a bit advanced for you, but this is the way to replace the missing spline with something else.
Fixing White Splines...
I got this a while back from James Moody.
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. All other procedures apply.
Need to change .texture file using PEV's Images2TGA.