Commuters in Tokyo are getting a chance to experience some cutting-edge display technology – although they might have to look hard to see it.

On three commuter trains some traditional paper advertisements have been replaced with colour e-paper displays. The displays, while technologically advanced, proved difficult to read during a demonstration on Thursday.

The 13.1-inch displays are made by Hitachi and based on Bridgestone Corp's "electronic liquid powder" technology. There are two displays on each of the three trains, mounted on the carriage wall at either end of the train.

Each display is connected to a smaller unit that contains companion electronics and an 8MB memory that can hold 37 advertisements. Each ad is shown for two minutes in a continuous cycle.

Electronic paper is still in its infancy. While it has been shown in prototype form for several years, the technology has only recently started making its way into commercial products. One of the highest profile uses is the Sony Reader electronic book, but it offers only a monochrome display. Colour e-paper development still lags behind.

The displays in the trains offer relatively low resolution, at 50dpi (dots per inch), and can only show eight colours. There's also no backlight, so on-screen images appeared quite dark and ads utilising dark colours, such as red and black, were quite difficult to read. Reflections on the display's protective plastic cover also caused problems.

The month-long tests are relatively high-profile because the trains in question run on the Yamanote Line, which runs in a loop around the centre of Tokyo. It's used by about 3.5 million people each weekday, making it one of the city's most important and heavily used railway lines.

The trial will gauge feedback from travellers and see how the system performs in a real-world environment, said Nobuyuki Ogura, a senior manager with Hitachi's transportation information systems division in Tokyo. They follow a trial last year in which a monochrome e-paper display replaced a wall poster at Tokyo Station, he said.

Hitachi will continue working on the screens and hopes to have a commercial panel ready for use by March 2008, he said.