The old route out of Broad Street to Dalston Junction is built for four
tracks throughout. Given that the ELLX is likely to only require two of
these tracks, does anyone know what the remaining formation will be
used for?

For that matter, how will the tracks themselves be positioned? One
smart thing to do would be to run them down the centre of the
formation, so that in case patronage begins to pick up significantly, a
set of outside loops can be built at the stations and new platforms
added, to permit non-stopping of trains.