Here is a picture from the N Gauge Society website of the proposed Queen Mary Bogie Brake Vans. I scratch built a model a few years ago, and have the NGS kit that I never built - but will definitely invest in a few of these!
Closing date for advanced (reduced price) orders is the 31st May 2011.
(http://i67.photobucket.com/albums/h287/tank_fv101/NGSBogieBrake.jpg)
Latest CAD drawings...
(http://www.ngaugesociety.com/uploads/images/RTRwagons/RTR3_QM_brake/QM_plated_w_sandboxes_for_web___small.jpg)