During the temporary timetable. The E400s fitted in nicely with the 1/1A/1B. So long as you avoided the low bridges. So probably would need to be a low height version.More eclispes on the one too.
Would help solve the overcrowding particularly at peak times.
Maybe more E400s for the 2 and 3 too.
That would allow a good number of E300s to be moved on.