What does this actually mean in practice on a proper working timetable? Will the doors still be released, but just heavily diminished rights in disruption and no prior advertisement? What about at a location where a train is scheduled to reverse?
It will depend on the service and local practice. A Thameslink or Southern service with an overnight unadvertised stop at Redhill will generally open the doors if it actually runs via Redhill, but clearly not if it goes down the Quarry Line. Southern typically will open doors at Selhurst, but Thameslink may not, including at reversal points. Sometimes it depends on whether the station is locked up or not.
There are instances where doors will not be released other than to drop off staff.
Clearly you take a risk in any instance that the train won't call at an unadvertised stop with no recompense available for any extra costs.
There were instances of unadvertised stops at both Ealing Broadway and Reading for the GWR diversions to Euston which very much weren't open to passengers.
