// genfit track
PndTrack *gtrk = (PndTrack*) gentrackarray->At(itrk);
// corresponding helix track
PndTrack *ltrk = (PndTrack*) recotrackarray->At(gtrk->GetRefIndex());
// with corresponding track ID
PndTrackID *ltrkID = (PndTrackID*) recotrackidarray->At(gtrk->GetRefIndex());

// from it you can get the MC track ID:  
int mcTrackID = ltrkID->GetTrackID();