.! $ROOTSYS/test/eventexe 1000 1 1 1 

TString dir = "$ROOTSYS/test/Event.root";
gSystem->ExpandPathName(dir);
const auto filename = gSystem->AccessPathName(dir) ? "./Event.root" : "$ROOTSYS/test/Event.root";

TFile oldfile(filename);
TTree *oldtree;
oldfile.GetObject("T", oldtree);

const auto nentries = oldtree->GetEntries();

Event *event = nullptr;
oldtree->SetBranchAddress("event", &event);

TFile newfile("small.root", "recreate");
auto newtree = oldtree->CloneTree(0);

for (auto i : ROOT::TSeqI(nentries)) {
   oldtree->GetEntry(i);
   if (event->GetNtrack() > 605)
      newtree->Fill();
   event->Clear();
}

newtree->Print();
newfile.Write();