Do you mean this one? It is at the site of a former railroad freight yard and includestwo roundhouses and turntables that used to be used for steam locomotives.
"German Technical Museum Berlin" - Deutsches Technikmuseum - German Technical Museum Tip by Nemorino
Actually it is quite central, just two stops on the U2 from Potsdamer Platz (get off at Gleisdreieck). But the immediate vicinity consists mainly of railroad tracks and old warehouses.
this link may help, Deutsches Technikmuseum - German Technical Museum, Berlin
For info in English (pages all seem to be in German) you may have to google appropriately e.g. 'U-Bahn museum Berlin'