Railway Divisional Superintendent Junaid Qureshi told APP that the bridge is intact but six rails of Up-track were damaged.
The CCPO Karachi Tariq Jamil told newsmen at Governor House that evidence of explosion at the site is being verified.
Prime Minister Shaukat Aziz told newsmen during the course of his press conference at the Governor House that as reported to him it was a minor explosion which did not cause any casualty except that some wagons of a passing goods train derailed.
The relief train was immediately rushed to the site of the derailment, which started removal of wagons from the down track.
PS Railway said that the track would be cleared in about next two hours after which railway traffic will be restored.
Railway officials said that the derailed wagons carried plastic beads, urea bags, and juice canes.
The DIG Operation Mushtaq Shah, who was present at the site, said it was not an incident of terrorism but investigation has started to find out the facts.
According to a press release issued by the Pakistan Railways in Lahore immediate relief arrangements were made and relief trains from Karachi and Kotri reached the site and started relief operation.
The front and rear portions of the train were taken to Landhi and Malir stations, respectively.
Eight Down (Tezgam), 36 Down (Faisalabad Express), 16 Down (Karachi Express) and 150 Down were terminated at Landhi station, whereas 42 Down (Karakoram Express) was terminated at Bin Qasim station. All these trains to the upcountry are being started from Landhi station after examination by the maintenance staff.
Buses were arranged immediately at Landhi station by the railway administration, and passengers brought to Karachi Cantt station along with their luggage. Similarly, passengers are being provided road transport to take them from Karachi City and Cantt stations to Landhi for boarding in the trains.
The driver and guard of the train said they heard a sound of the blast and then the derailment took place. An inquiry has been ordered in this regard, the release said.