Carl Greatrix (Bricktrix) has posted a really great Transfer Post Office (TPO) car over on Eurobricks. Not only is the car a great looking model, it also features a working pick-up system for the mail bag.You can see the complete model and explanation on Bricktrix's Flickr stream.
On a related note, if you are looking for prototype photos of similar cars, there's a nice Railway Post Office (RPO) group on Flickr now, featuring photos dedicated to this unique form of postal pickup and delivery.
