"tim....." wrote in message
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"John Salmon" wrote in message
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"tim....." wrote
"Peter Masson" wrote

Presumably bidirectional signalling, to make single line working
practicable while the other line is under maintenance.

Nice idea, except that HSE rules don't allow such operations.

No such rule applies between Northallerton and Newcastle, where
late-night passenger trains regularly run 'wrong line' under SIMBIDS.

I was referring to the bit about working on an adjacent line.

Which is exactly what happens on the ECML. Also, there was recently
(January) a two-week block of one track between Totnes and Plymouth for
relaying, whilst trains continued to run on the other track. I repeat,
there is NO overall prohibition of this practice.