An error on a signaling system under test on Shanghai's Metro Line 10 caused a train to take a wrong turn, carrying passengers on the wrong branch after the line split at Longxi Road, operators said yesterday.

The train, which should have been heading for Hangzhong Road after Longxi Road, instead took the other line on the Y-shaped intersection and ended up at Hongqiao Railway Station at around 7:20pm on Thursday.

Passengers were told by Metro staff to take trains back to Longxi Road. An announcement over the train's loudspeakers said a signal problem had caused it to take a wrong turn.

Some passengers said the train had even begun to run backward for a short distance after it stopped at Shanghai Zoo, the first station on the wrong route.

"I was reading a newspaper on the train when it stopped for one minute with none of its doors opening," said a 25-year-old passenger surnamed Gu. "Then all of a sudden it started to run backward for a few seconds, which scared some people."

The incident raised fears about collisions with other trains on the branch line, some passengers later complained on microblogs. They began to compare the incident to Saturday's high-speed train crash, which was said to have been caused by defective signaling equipment.

Metro operators said an error occurred when they were upgrading and testing the new signaling system on Line 10 at 7:06pm.

"A sudden block of information flow hit the system that sent a false signal to the train," said a Metro operator surnamed Lan.

Lan said the operators, the train driver and also the signal protection system immediately detected the problem and raised the alarm, but they could not turn the train back as it was already headed in the new direction.

Lan said they had been testing with empty trains but needed to gather more information with certain passenger flows in order to debug the system.

He said the testing was safe because all the trains were equipped with signal protection systems and an automatic train control system which would prevent collisions.

The new Communication Based Train Control system will be widely used on Metro lines after testing to help prevent accidents as it is currently the most advanced signaling system in the world, Lan said.

Metro operators said Shanghai's subway train signaling equipment supplier was not the same company said to have supplied the signaling equipment that an initial investigation had blamed for the deadly bullet train collision on July 23 in Wenzhou, Zhejiang Province.