I live in Japan. The only places I’ve seen single car trains like this is long lines (2+ hours) in small / rural areas. They generally have one every 30-45 minutes and pick up smaller amounts of people. I’ve seen them joined during summer rush times butnothing longer than 2-3 cars. Its great for that kind of flexibility. Low wait times during off season, higher capacity during peak travel times. During the off season the trains are used “just as much” but in different locations. Kind of like load balancing.
This will eventually stop working, so isn’t a long-term solution, sadly.