I believe the current generation of wheels are an improvement on older ones, and the rack certainly adds stability to the folded package, but I've never found trolleying a Brompton around on its easy wheels to be quick and controllable enough for a tube-station-like environment. With four (or presumably three, on the L type) wheels in contact with the ground, it only wants to roll in a straight line. Your instinct to lean it to steer, as you might a skateboard, just causes it to tip. The best method I've found it to pick it up by the saddle and push it along on two wheels.
(If you've got luggage attached and pulling it by the handlebars as a shopping trolley, it's tolerable, as you aren't trying to maintain a brisk walk.)