TOKYO -- The operator of a busy Tokyo-area rail line is offering riders up to 4,000 yen ($38) worth of annual rewards points for avoiding express trains during the city's notoriously congested morning rush hour.

The rewards program begins in July, according to rail group Keikyu. To participate, riders need to install an app on their smartphones.

The app can "hear" special audio messages broadcast in conjunction with on-board announcements. Though inaudible to human ears, these messages tell the app whether the user is riding a local or express train.

The railway operator wants to redirect passengers to less-crowded local trains between 7:30 and 9 a.m. on weekdays to avoid congestion and delays.

The rewards program will apply to rides from Heiwajima Station to Shinagawa Station, Keikyu's main hub in central Tokyo. Local trains make seven stops on a trip that takes 15 minutes, while express trains make only three stops in half the time.

With Tokyo preparing to host the 2020 Summer Olympics, Japanese companies are looking for ways to cope with congestion in the capital. Some have gone as far as planning to close their headquarters during the games for all but essential staff.