In message , Clive D. W. Feather
writes
Alternate working requires a sufficiency of crossovers to switch
trains. The tube lines don't have these, in general.

A design fault, then.

There's also a problem with trainstops - they need to be lowered to
prevent a train running the wrong way from "back-tripping". Except
where bidirectional working is allowed (e.g. at termini) this isn't
designed into the signalling.

Another fault.

[These are excusable when the tubes were originally designed for normal
working hours. I think we forget how little of life was 24x7 as recently
as 30 years ago. I could certainly drive from Cambridge to Chelmsford at
midnight and see no other car on the road. At some point this changed
radically - perhaps the tubes need to keep up.]
--
Roland Perry