Although it is not widely published as like in Europe, there are pickpockets in Japan too. May be it is much more easier to steal someone's wallet or purse in Japan than in Europe.
Anyway, the situations in Japan are different from in Europe. In general, thieves in Europe target foreign tourists rather than domestic people. Therefore pickpockets on trains usually happens to foreign tourists whom are easily to be targeted by criminals.
However, Japanese thieves lock on to Japanese who are sleeping, drunk or careless. Most of the pickpockets happen on last train where there are many passengers who are drunk and sleeping on the train or at crowded places such as public gambling venues for horse racing, boat racing, cycle racing-keirin and auto racing.
On the other hand, Japanese are not careful to carry their valuables. For thieves, it should be easier to steal their valuables than in Europe.