Well, he started off sitting next to her, simple enough. But then he was running in the air, the carriage lashed to his back as he pulled it along the sky. Later still, while she’d just turned to glimpse back at her pursuer, he'd become the steam engine itself, pushing the pistons by squatting and standing up again. All while telling her how simple it all was, of course. And then she joined him, working the other piston, too!