gistfile1.js

var interval = 6000;
var g = this.group;
var fall = true;

var callBack = function(){
	return function(){
                //Making it alternate between falling and going all the way back to the top instantaneously
		if(fall){
                        //Falling
			g.transition().duration(interval).ease('linear').attr('transform', 'translate(0,3000)');
			fall = false;
		}else {
                        //Resetting
			g.transition().duration(0).attr('transform', 'translate(0,-1000)');
			fall = true;
			interval = 6000;
		}
		d3.timer(callBack(),interval);
                //Timer also executes the function until it returns true
		return true;
	};
};

//Timer executes the specified function after the specified number of seconds has passed
//In this case, I want it to start falling immediately
d3.timer(callBack(), 0);