It's the ships.tbl, remember it goes in the $afterbuner:, as taken from the WIKI:
# $Trails:
* Implemented before FS_Open 3.6.5
* Allows afterburner trails to be used
* +Bitmap:
o Texture used to draw the afterburner trail
o Syntax: Filename
* +Width:
o Width of afterburner trail at the beginning of it (ie. the thruster end)
o Syntax: Value, (in meters)
+ Default: 1.0
* +Alpha:
o Transparancy of the trail, value from 1 (non-transparent) to 0 (totally transparent)
o Syntax: Value
+ Default: 1.0
* +Life:
o How long the trail will last
o Syntax: Value, (in seconds)
+ Default: 5.0
It's not "$Trail:", that's for normal (non-afterbuner) trail. The ones in Species.tbl are for thrusters and their glows, depending on ship's motion (stop/accelerate/etc.).
