> But yes, no ability to single-track is an issue.
Though it's not an absolute panacea, either. The Washington subway has crossovers every two to three stations and is set up for bi-directional operation on one track, and the two things I immediately noticed when I visited Washington eight or so years ago were:
- service levels on weekends were already only meh even without construction works
- there were construction works on a very small segment of the Red Line between two of those aforementioned crossovers – those works only required closing one track, so they decided to continue running a service on the remaining track. Unfortunately single-tracking severely limits the number of trains you can run, so that tiny closure meant that on that weekend the whole length of the Red Line (all 32 miles of it!) was only operating every 20 minutes or so (originally I had considered an accommodation along the Red Line, luckily I had reconsidered and chosen a different place to stay even before learning about those construction works).
After some googling, it seemed that this (i.e. construction works on a tiny stretch of track significantly affecting the service of the whole line even during the daytime, at least at weekends) was not an uncommon occurrence in Washington.
While single-tracking might be no problem late at night (when a reduced frequency doesn't matter that much, and might not be even all that much reduced as compared to normal service levels at that time of the day), or during unplanned short-term disruptions, for that kind of scheduled-in-advance all-day construction works I'd say it's at least debatable whether a complete closure of the affected stretch of track (with a bus replacement service) and a full service on the whole rest of the line might not be a better compromise…