In message , at 09:37:28 on Sat, 1 Jun 2019,
Anna Noyd-Dryver remarked:

if it was closed for engineering works they wouldn’t reopen it for
three trains after midnight and four trains before 0630 every day,

Those sound like stock positioning moves, which they also schedule for
passenger service.

However it means the line is open and available during those hours. If it
wasn’t, they’d have to go the long way round.

It's the GA announcement which gave the impression the line was closed
(for engineering) during the periods buses were being supplied.

"[because of] a major upgrade to ... the track that connects
Stevenage with Watton-at-Stone"
--
Roland Perry