public function getTrackEngineClasses()
    {
        static $dummyClasses;

        if (! $dummyClasses) {
            $dummyTrackData['gtr_id_track'] = 0;

            foreach ($this->getTrackEngineClassNames() as $className) {
                $dummyClasses[$className] = $this->_loadClass('Engine_' . $className, true, array($dummyTrackData));
            }
        }

        return $dummyClasses;
    }