A government bus headed to Jadavpur from Howrah climbed the median on Chowringhee Road, ploughing through the iron railings near Ispat Bhavan on Monday morning.
At least five passengers were injured and taken to SSKM Hospital, about 1km away. They were provided first aid and then discharged.
Eyewitnesses told police the West Bengal Transport Corporation bus was moving towards Chowringhee Road's intersection with Middleton Street when it dangerously veered towards the divider.
The bus's right side hit a wrought iron pole on the median divider and ripped apart the iron railings before coming to a halt.
"Some bystanders said the driver possibly realised that the bus had developed a snag and deliberately moved towards the median divider to bring it to a halt after hitting the iron railings," a senior police officer said.
"The incident occurred around 10.20am. Since Monday was a state holiday, traffic was relatively light and the bus wasn't carrying many passengers.
The front of the bus was twisted under the impact of the collision. Concrete chunks of the median came off.
The bus came to a halt with a part standing on the divider and the remaining tilted.
"The incident took place on the Exide-bound flank of the road, which was nearly empty. If this had happened on a normal working day, several people could have been injured," the police officer said.
A close inspection of the bus's front revealed the tie-rod, which holds the balance of the two front wheels, had come off and hung loosely.
A section of mechanical experts said a bus's tie rod helps the vehicle to turn when the steering is moved, and if this rod gets worn out, the steering's function may be affected.
"We have shifted the bus to Shakespeare Sarani police station so that forensic experts can conduct a mechanical test on the vehicle on Tuesday," a senior officer of the police station said.
"Going by the accounts of the eyewitnesses, it appeared the bus driver realised something was wrong and chose to hit the median to bring the vehicle to a halt."
The accident again brought under the scanner the condition of buses on Calcutta's roads. They are ill-maintained and officials in charge of the job have little accountability.
The version offered by the police in this case suggested the driver may not have been at fault.
But it is also common knowledge that government buses are among the more reckless on Calcutta's congested roads.
The police used a crane to lift the bus after removing the iron railings.