How to create an endless track?

 //object prefab for clone
 public var platform : GameObject;
 
 //take this object into Camera you can do it (Drag this object drap onto Camera.
 //object for spawn object
 public var spawnPosition : Transform;
 
 //one more position where platform going to destroye.
 //also take this object in main Camera to
 public var destroyPosition: Transform;
 
 //set distance for next spawn
 public var spawnDistance : float;
 //this is automatic access do not change
 private var lastPlatform : GameObject;
 
 function Start() {
     //Instantiate on start
     lastPlatform = Instantiate(platform, spawnPosition.position, Quaternion.identity) as GameObject;
 }
 
 function Update() {
     //make distance between 2 platforms
     if (lastPlatform.transform.position.y + spawnDistance < spawnPosition.position.y)
             {
                 //call one more function to getrate platforms
                 InstantiatePlatform();
             }
     //destroye if platform position lower then screen height
     if(lastPlatform.transform.position.y < destroyPosition.position.y) {
         //Destroy object 
         Destroy(lastPlatform);
     
     }
 }
 function InstantiatePlatform()
         {
             lastPlatform = Instantiate(platform,new Vector3(Random.Range(-13,13) * 0.2f,spawnPosition.position.y,1),Quaternion.identity) as GameObject;
         }