"1577+2260" wrote

So there are two options for down trains:

* remain on the relief lines at Hayes, and use the Up Airport flyover.
This involves running on the UA line between Airport Junction (11m14c)
and Heathrow Tunnel Junction (12m27c), i.e. blocking the line (with
its 4tph HEx and 2tph HC coming the other way) for over a mile.

* cross to the down main at Southall East, in the process taking a
stopping path on the down main and blocking the up main when crossing
over.

AIUI the first one is the one that is to be used. Although it's better
than the crossing move and stopping on the fast lines, it still seems
quite a potential constraint - 6tph up vs 2tph down over that mile is
asking for trouble once things are running out of course.

The June MR confirms your understanding, and gives some details of how
Airport Junction would be modified for Crossrail. Basically, the Up Airport
flyover would become the Down Crossrail, and a new Up Airport flyover would
be built right across to the north of the layout, with a ramp down for Up
Crossrail trains, and a flyover back to the middle of the layout for Up
Heathrow Express. Hayes & Harlington would be rebuilt, with the Up Relief
platform becoming an island, with Up Crossrail Airport trains using the
outer face.

As well as the bidirectional working of part of the Up Airport line, Up
Heathrow Connect trains will, when they're allowed back to the Airport, have
to cross the Down Relief on the flat.

Peter