diff --git a/python/rootUtils.py b/python/rootUtils.py
index b31bee3210..82a0539d4e 100644
--- a/python/rootUtils.py
+++ b/python/rootUtils.py
@@ -81,14 +81,30 @@ def errorSummary():
     print("Summary of recorded incidents:")
  for e in _error_log:
     print(e, ':', _error_log[e])
+
 def checkFileExists(x):
-    if x[0:4] == "/eos": f=gSystem.Getenv("EOSSHIP")+x
-    else: f=x
-    test = TFile.Open(f)
-    if not test:
-       print("input file",f," does not exist. Missing authentication?")
-       os._exit(1)
-    if test.FindObjectAny('cbmsim'):
-     return 'tree'
+
+    if isinstance(x, list):
+        print("I'm a list")
+        for _f in x:
+            if _f[0:4] == "/eos": f=gSystem.Getenv("EOSSHIP")+_f
+            else: f=_f
+            test = TFile.Open(f)
+            if not test:
+               print("input file",f," does not exist. Missing authentication?")
+               os._exit(1)
+            if test.FindObjectAny('cbmsim'):
+             return 'tree'
+            else:
+             return 'ntuple'
     else:
-     return 'ntuple'
+        if x[0:4] == "/eos": f=gSystem.Getenv("EOSSHIP")+x
+        else: f=x
+        test = TFile.Open(f)
+        if not test:
+           print("input file",f," does not exist. Missing authentication?")
+           os._exit(1)
+        if test.FindObjectAny('cbmsim'):
+         return 'tree'
+        else:
+         return 'ntuple'
diff --git a/shipgen/MuonBackGenerator.cxx b/shipgen/MuonBackGenerator.cxx
index 25988d0844..e5bde3a878 100644
--- a/shipgen/MuonBackGenerator.cxx
+++ b/shipgen/MuonBackGenerator.cxx
@@ -13,6 +13,7 @@
 #include "FairPrimaryGenerator.h"
 #include "ShipMCTrack.h"
 #include "ShipUnit.h"
+#include "TChain.h"
 #include "TDatabasePDG.h"  // for TDatabasePDG
 #include "TFile.h"
 #include "TMCProcess.h"
@@ -38,12 +39,18 @@ MuonBackGenerator::MuonBackGenerator()
 Bool_t MuonBackGenerator::Init(const char* fileName) {
   return Init(fileName, 0);
 }
-// -----   Default constructor   -------------------------------------------
-Bool_t MuonBackGenerator::Init(const char* fileName, const int firstEvent) {
-  LOG(info) << "Opening input file " << fileName;
-  fInputFile = TFile::Open(fileName);
-  if (!fInputFile) {
-    LOG(fatal) << "Error opening the Signal file: " << fileName;
+
+Bool_t MuonBackGenerator::Init(std::vector<const char*> fileNames) {
+  return Init(fileNames, 0);
+}
+
+Bool_t MuonBackGenerator::Init(std::vector<const char*> fileNames,
+                               const int firstEvent) {
+  LOG(info) << "Opening input file " << fileNames.at(0);
+  TFile testFile(fileNames.at(0));
+  auto testKeys = testFile.GetListOfKeys();
+  if (testKeys == nullptr) {
+    LOG(fatal) << "Error opening the Signal file: " << fileNames.at(0);
   }
   fn = firstEvent;
   fPaintBeam = 5 * cm;  // default value for painting beam
@@ -51,9 +58,14 @@ Bool_t MuonBackGenerator::Init(const char* fileName, const int firstEvent) {
   fPhiRandomize = false;      // default value for phi randomization
   fsmearBeam = 8 * mm;        // default value for smearing beam
   fdownScaleDiMuon = kFALSE;  // only needed for muflux simulation
-  fTree = fInputFile->Get<TTree>("pythia8-Geant4");
-  if (fTree) {
+  if (testKeys->FindObject("pythia8-Geant4")) {
+    fTree = new TChain("pythia8-Geant4");
+    for (auto& f : fileNames) {
+      LOG(info) << "Opening input file " << f;
+      static_cast<TChain*>(fTree)->Add(f);
+    }
     fNevents = fTree->GetEntries();
+    LOG(info) << "Reading " << fNevents << " entries";
     // count only events with muons
     fTree->SetBranchAddress("id", &id);  // particle id
     fTree->SetBranchAddress("parentid",
@@ -82,9 +94,14 @@ Bool_t MuonBackGenerator::Init(const char* fileName, const int firstEvent) {
     }
   } else {
     id = -1;
-    fTree = fInputFile->Get<TTree>("cbmsim");
+    //    fTree = fInputFile->Get<TTree>("cbmsim");
+    fTree = new TChain("cbmsim");
+    for (auto& f : fileNames) {
+      LOG(info) << "Opening input file " << f;
+      static_cast<TChain*>(fTree)->Add(f);
+    }
     fNevents = fTree->GetEntries();
-
+    LOG(info) << "Reading " << fNevents << " entries";
     // Detect format by checking branch name:
     // STL format uses PlaneHAPoint, TClonesArray uses vetoPoint
     TBranch* mcBranch = fTree->GetBranch("MCTrack");
@@ -122,6 +139,12 @@ Bool_t MuonBackGenerator::Init(const char* fileName, const int firstEvent) {
   }
   return kTRUE;
 }
+
+// -----   Default constructor   -------------------------------------------
+Bool_t MuonBackGenerator::Init(const char* fileName, const int firstEvent) {
+  std::vector<const char*> fileNames = {fileName};
+  return Init(fileNames, firstEvent);
+}
 // -----   Destructor   ----------------------------------------------------
 MuonBackGenerator::~MuonBackGenerator() {
   if (!fUseSTL) {
@@ -228,8 +251,8 @@ Bool_t MuonBackGenerator::ReadEvent(FairPrimaryGenerator* cpg) {
       }
     }
   }
-  if (fn > fNevents - 1) {
-    LOGF(info, "End of file reached %i", fNevents);
+  if (fn == fNevents) {
+    LOGF(info, "End of tree reached %i", fNevents);
     return kFALSE;
   }
   if (fSameSeed) {
diff --git a/shipgen/MuonBackGenerator.h b/shipgen/MuonBackGenerator.h
index 169477129c..f3a297804f 100644
--- a/shipgen/MuonBackGenerator.h
+++ b/shipgen/MuonBackGenerator.h
@@ -27,10 +27,13 @@ class MuonBackGenerator : public FairGenerator {
 
   /** public method ReadEvent **/
   Bool_t ReadEvent(FairPrimaryGenerator*);
-  virtual Bool_t Init(const char*, int);  //!
-  virtual Bool_t Init(const char*);       //!
-  Int_t GetNevents();                     //!
-  void CloseFile();                       //!
+  virtual Bool_t Init(const char*, int);               //!
+  virtual Bool_t Init(const char*);                    //!
+  virtual Bool_t Init(std::vector<const char*>, int);  //!
+  virtual Bool_t Init(std::vector<const char*>);       //!
+
+  Int_t GetNevents();  //!
+  void CloseFile();    //!
   void FollowAllParticles() { followMuons = false; };
   void SetSmearBeam(Double_t sb) { fsmearBeam = sb; };
   void SetPaintRadius(Double_t r) { fPaintBeam = r; };