Train track (4)
I believe the answer is:
rail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">train track</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for rail that I've seen before include "Protest strongly (at)" , "Bar; waterside bird" , "Supported bar - inveigh" , "The permanent way" , "British --; picture --" .)