Im doing something similar to the light trail project you posted and am having some weird problems.
I have a person drawing imaginary lines in the air over 2 secs. i have created a null and tracked their finger to its position property, then pickwhipped your project file "particle trail" layer to the null.
It works but my lines are not smooth, rather a series of short straight lines that make up curves. i used the smoother as you suggested but this did not help me. i made all my KFs bezier, but again this did not help.
Any idea on what i could be doing wrong without seeing my footage.