Hallelujah
As unusual as it seemed, Lex's emotions were not so exaggerated that he would end up doing something illogical. The reason he ran with the cart lifted above him instead of driving normally was because his short burst speed was much greater than the cart itself.
Before long, he leaped up into the air and got back in the driver's seat, allowing the cart to fall back onto the platform. The cart swerved a bit and nearly fell off the moving train, but Lex managed to regain control.