"Brian Watson" wrote

I found myself thinking, "why start at Shenfield?"

Is there so much demand on that line for such a route, or capacity further
up it (at Chelmsford for example)?

The GEML is 4-track as far as Shenfield, with the Slow (Electric) [1] Lines
used for the stopping Metro service, most of which will be diverted to
Crossrail. To run Crossrail trains beyond Shenfield would impinge on
reliability. In the same way, on the GWML Crossrail trains will normally be
confined to the Relief (Slow) Lines.

[1] In the original Shenfield electrification only the Slow Lines were
wired, and named the Electric Lines. Subsequently all four tracks have been
wired.

Peter