The Keio spokesman said the company had received 351 complaints about groping in the year up to March 2000 - an increase of nearly 100 over the previous 12 months.
"There have been problems with women being groped on the trains, and we felt this was a problem that should be dealt with," said a spokesman for Keio Electric Railway Company, which operates trains from central Tokyo to the capital's western suburbs.
"Given that the holiday season is especially crowded, we thought it would be a good time to test the idea of separate carriages," the spokesman said.
December is marked by "Forget-the-Year" parties - the Japanese equivalent of Christmas parties - and late-night trains are crammed with drunken, boisterous male commuters.
Serious problem
Groping is a serious problem for Japanese women on the long commute home, where they are often forced into close proximity with men on packed subways and commuter trains.
The Keio spokesman said the company had received 351 complaints about groping in the year up to March 2000 - an increase of nearly 100 over the previous 12 months.
The real figures are likely to be much higher, as the vast majority of cases go unreported.
A company survey found that 17% of female passengers said they had been molested at least once.
Keio said that, if passengers responded positively to the service, it would consider introducing the system on a permanent basis.
This is the first female-only train carriage in Tokyo since 1973, but a railway service in Osaka has run special carriages for female high school students since 1954.
Japan's Ministry of Labour received 9,500 reports of sexual harassment last year.