I downloaded the Terrace spline and found out that there's an updated built-in version in TRS22/Plus at build 3.5 <kuid2:133671:11009:2> Terrace 04 spline, but this version isn't available for TRS19!
I then checked your version <kuid2:133671:11009:1> Terrace 04 spline in TRS19 and I cannot get any track to attach the spline.
I checked the config.txt file and saw everything set up properly. My recommendation is to redownload the asset and see if that fixes the problem.