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Was on it at the time on one of the public days - from memory and looking back through old photos and videos it almost certainly wasn't a mockup, given that practically all other vehicles outside were real, it had all underfloor equipment installed and was placed on one of the exhibition tracks...
see 58:35: "[...] two of the ideas where the bogies might be accommodated [...] that was produced by PriestmanGoode for TfL in a design study. Having come up with some ideas we invited the industry to look at our ideas and come back with their own ideas [...]". the piccadilly hasn't been...
on the few that i can find which clearly show both middle carriages (e.g. https://www.vagonweb.cz/fotogalerie/foto/202203/20220922_180509.jpg) they do have the extra door.
four-car 756s (are supposed to) have 158 seats, 231s have 170 (see here). the difference is because the 756s have an extra door which takes up the same amount of space as three rows of seats, not because there's any difference in table provision (both have 21).
Stadler's published a datasheet (PDF) for the units. unsure how much of the information in there is actually new, but it makes for a good overview of their dimensions, capabilities and interior layout.
definitely AC motors. (modern trains convert from AC to DC (if their power supply is AC), then back to variable-frequency AC to power the motors themselves; it's that first stage (the AC->DC transformers) whose installation would require removing the batteries.)
Hey, at least those actually entered service on time, with the exact service that was agreed upon more than a year ago, and have apparently done pretty well since.
Alongside the 756's there are also the 230s for Wrexham - Bidston and the Class 398s (which unlike the 756 and 230 don't have any diesel engines, making them arguably "purer" battery EMUs). The future Tyne and Wear units will also have batteries for depot shunting and could have more batteries...
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