Derry/Londonderry has an hourly service to Belfast, though, which seems rather more frequent than many of the examples on this thread. I did see it claimed that it's the busiest (in terms of passenger numbers) stretch of single-track railway in the UK, and it does feel eye-wateringly slow (2 hours from Belfast), mind!Or cases like Chemnitz or Derry~Londonderry, which are at least reasonably sized cities in countries with generally good coverage but a relatively limited or no intercity service themselves.
Does Luxembourg (city) have true intercity trains, or just regional services that happen to link bigger places?