This week's Private Eye pondered what could take place to cope with the extra capacity, as each train will have seven carriages instead of today's six.
"The new trains will be designed so the last or first four doorways, or a combination of both, can stay locked when Circle line platforms are shorter than the new trains. Regular travellers will adapt; but the Circle is heavily used by visitors, many travelling from mainline stations with luggage. What will happen when they realise the train is at their station and the nearest open door is a carriage away along a crowded aisle? Cue mayhem."
They've got a point. Occasionally I come into Charing Cross and the Bakerloo Line train there doesn't open just one set of doors. So with more doors not opening, it could be a bit of a scramble for tourists & people may easily stand at the doors waiting for them to open. Especially if they don't understand or hear the announcements properly.
Private Eye continued "Platforms aren't LU's only headache. Lots more passengers will change trains at poorly-equipped Edgware Road station. Tracks at Hammersmith station need costly extension for seven carriages. Today many trains are berthed in sidings around around the Circle; unless LU can find big money to extend the sidings for seven carriages, it will waste resources shifting trains to and from sidings in suburban depots.
Metronet's promised new signalling for "sub-surface" lines (including also the Metropolitan and District) would have allowed extra trains on the Circle, potentially increasing Circle capacity with six-carriage trains until infrastructure was ready for seven carriages. Installation of new signalling should be happening now, but the contract was scrapped after Metronet went bust."
Thanks to Jon Justice for spotting this. It looks like there's a number of challenges in the Circle Line's transformation, the least of which will be dealing with pedants saying "but it's not a Circle any more", when the changes were announced.