Mystery surrounds bullet trains serving the newly built high speed railway between Shanghai and Beijing, after they began month-long tests yesterday with some carriage interiors hidden by curtains.

A CRH 380A bullet train arrives at Shanghai Hongqiao Railway Station yesterday. The CRH 380A, taking part in full-route tests of the Shanghai-Beijing high-speed railway, is the world's fastest train, with a top speed of 468 kilometers per hour. When the service begins next month, the train will pull 16 carriages and have a passenger capacity of 1,004.

Shanghai Hongqiao Railway Station, the major terminal for the service, saw the first train in the test leave at 8:45am. In total, 18 services departed from the Hongqiao station yesterday, with the last leaving at 5pm.

On the trial trains, curtains were apparently covering "VIP carriages." Earlier reports said at least some VIP carriages would be removed, creating space for more standard seats.

This follows criticism that China's high-speed rail services are too expensive for many ordinary travelers.

Reporters were not permitted to examine the trains yesterday and passengers will not be carried during trials.

Mindful of the high-speed service's rival for passengers between Shanghai and Beijing, the rail authorities insisted that the service will "be comparable to that on airplanes."

Indeed, a glimpse into a business suite carriage revealed plush red seats, complete with a side light and an impressive array of buttons on the armrests.

During yesterday's test run, the fastest journey along the 1,318-kilometer route from the Hongqiao station to Beijing South Railway station took four hours and 48 minutes at a speed of 300kph, officials said.