PIE base ‘to draw, drag, move’. Not in The American Heritage Dictionary of Indo-European Roots.
Other derivatives include tract ‘area of land’, attract, contract, detract, distract, extract, protract, retract, and subtract, from Latin trahere ‘to draw’, and treat, treaty, and tract ‘short treatise in pamphlet form’, from tractare ‘to handle’, frequentative of trahere ‘to draw’.
[Pokorny trāgh-, trō̆gh-, and trē̆gh-, pp. 1089.]