Ember unable to access dynamic segment parameter

I have my Router.js defined as below;

this.route('step1', function() {
      this.route('step2dynamic', {path: '/:dynamic_step_name'}, function(){
            this.route('step3a');            
            this.route('step3b');
      });
});

Now in one of my routes, I do

this.transitionTo('step1.step2dynamic.step3a', 'step2DynamicValue');

In my routes\step1\step2dynamic\step3a.js, I have the model hook defined as

model: function(params) {
    // Why is params not contain the passed parameter value ?
}

Here I am not getting the params.dynamic_step_name. It is an empty object. What am I doing wrong here ?