The train form with two locomotives sandwiching a fixe set is based on British Rail's HST. Okay, there are or were earlier examples than that, including Britain-s Blue Pullman, Germany's TEE and others. But I think there is no denying that the British HST went a long way to making this type of train popular and succesful.
As ordered, the BR versions came with both 7 and 8 passenger cars, depending on the route. Furthermore, initially there was both a buffet car and a full restaurant, at least on the versions earmarked for longer trips. Full restaurants were phased out a few years later, replaced by meals served at your seat in the first class on longer trips. Over the years cars have been rebuilt and repurposed and indeed cars have been rebuilt from the general fleet to strengthen the HSTs. with many sets being one or even two cars longer than initially delivered. On the other hand parcels are no longer carried, so the sets today are quite different in many respects to the configuration as first designed. Not to mention they have also been re-engined.
BR and its privatized succesors were fortunate in that there were plenty of Mk3s available on other services that could be rebuilt for HST use (basically buffers removed and rewired for diferent voltage). I hope Brightline has some sort of arrangement with Siemens to be able to add further cars in the longer term future without being charged some extortionate price,as happened to Amtrak when they tried to strengthen the Acela sets.
