32-515Z Derby Lightweight 2-Car BEMU 'Test Unit Gemini' BR RTC (Original)EXCLUSIVE TO RAILS OF SHEFFIELD Derby Lightweight 2 Car DMU 'Test Unit Gemini' BR RTC (Original) This model is based on Bachmann's original 2011 Derby lightweight DMU tooling. Our BEMU model is as close as possible to replicating this unique prototype as the tooling will allow. Prototype History The British Rail BEMU was an experimental two car battery electric multiple unit (BEMU), converted from the prototype Derby Lightweight Diesel multiple units.