I used to travel semi-regularly on the South Wales main line and think a 5 car IET would be adequate for most of the day (outside the peaks) - The issue is Cardiff Central. There is a large volume of trains that use all platforms 1-4 and extra dwell time of detatching a unit before sending it off to Canton/another platform isn’t really ideal. Might work if planned well but could cause issues as soon as there’s any kind of disruption. There isn’t anywhere else on the route that would be suitable to add/drop coaches either, so they are probably better staying in full formation to Swansea for the time being.
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