Several weeks ago, JR's Saikyo Line started to reserve at least one car on its nightly commuter runs for women. The move followed a precedent set last year by the Keio Line, whose new service, according to reports, is very popular.
Though it sounds like a trend in the making, it's actually a return to the past. After the war, many commuter trains had gender-segregated cars. The reasoning at the time had to do with the prurient idea that crowded trains were unbecoming places for women. Feminists eventually stopped the practice, though, arguing that it was discriminatory.
The move back to segregation was brought on by increasing intolerance for the long-standing problem of chikan, men who grope women on trains. What's interesting about the change is that men in general have welcomed the new segregation since separate cars makes it less likely that innocent men will be accused of molesting women.