I’m surprised platform 10A hasn’t been mentioned on the GEML at Stratford. I’ve only ever seen it used during disruptions (usually as a last minute platform change) which results in everyone running down the stairs to the subway and back up again!
Are there any booked workings to use platform 10A?
9, 10 and 10A are (or perhaps were, given current reductions) what you might describe as tidal. In the morning peaks some up trains call alternately at 9 and 10 as this allows a train to arrive in 10 whilst the one in 9 is just leaving. Some down trains will then use 10A. In the evening peaks down trains can call alternately between 10 and 10A and up trains use 9. This was certainly happening pre-Covid as I caught plenty of trains for Southend Vic from 10A if I'd been to our work's Stratford office, and RTT seems to suggest it still is, several trains in the morning and evenings are booked for 10A. Off peak this doesn't happen, in general down freight off North London uses 10A, up uses 10, and none runs through in the peaks at all.
How did it end up being numbered 10A rather than being numbered in the general run?
Before they put the island platform in (which is the current 9 & 10), there were two platforms 9 & 10 and two fast lines between those and 5-8 (what is now the TfL side), anything that stopped at Stratford generally used 5 & 8 and very little stopped at 9 or 10. When the new island was built to cater for the growing Docklands traffic, 11 & 12 (Cambridge line) either did or were due to come back into more regular use too, so it made sense to just renumber 10 to 10A and 9 to 10. I think that makes sense...