The Suica card is incredibly versatile, but it cannot be used on literally every train in Japan. It is valid on all JR East lines in the Tokyo area, as well as subways and buses. Thanks to Japan's nationwide IC card integration, it also works on major transit systems in cities like Osaka (ICOCA area), Kyoto, and Sapporo. However, you generally cannot use it for long-distance Shinkansen (bullet train) travel without pre-registering it with a digital service like "Smart EX." Additionally, it cannot be used for continuous travel between different IC regions (e.g., you can't tap in at Tokyo and tap out at Osaka). In rural areas, some private local lines still only accept cash. While it is the "gold standard" for city commuting and short-haul trips, you will still need to purchase paper tickets or use specific apps for most intercity express travel.