I was actually really surprised when I first found out that subway systems in many large systems shut down at night. I live in a modestly-sized city (Dresden, Germany, ~500k inhabitants), and the local transport authority prides itself on operating nearly the entire tram network (plus some important bus services) 24/7. Just this year, they have actually increased the schedule density during weekend nights. Trams now run every 30 minutes instead of every 60 minutes (which is still the rhythm on weekday nights).