As an alternative to the ECML sleeper proposal (thread), how plausible could overnight intercity services be, with the main purpose being to keep people moving overnight without necessarily being quick, and without providing accommodation? They could either only serve major stations so engineering works can be easily avoided if necessary (e.g. no Grantham, Newark or Retford – I know that's a bad example as Peterborough – Sleaford already isn't a through service on Sundays, but hopefully I've communicated the principle) or just being cancelled for engineering works and maybe serving stations with few or no regular long-distance services (e.g. the existing 18:13 Saturday service from Crewe to Berkhampstead and Hemel Hempstead).
For comparison's sake, I think this is the most recent map of overnight ICEs in Germany.
For comparison's sake, I think this is the most recent map of overnight ICEs in Germany.