There are new safety fears on London Underground's (LU) Central Line just months after it re-opened following a derailment, BBC London has learnt.
All Central Line trains are being tested
Urgent checks are being carried out on the entire fleet of trains after cracks were found on brackets which hold motors to the underneath of the trains.
The Rail Maritime and Transport (RMT) union called for trains on the Central Line to be taken out of service, so that detailed checks could be made. But LU said they were still safe to run.
The line was closed earlier this year after a derailment at Chancery Lane station, which led to safety modifications on all 85 trains using the line.
Bobby Law, the RMT's senior London official, said 50 trains were being checked by LU following the discovery of cracks.
He said all 50 should be taken out of service while the checks were being made and train speeds should be drastically reduced.
Defective bracket found after testing
More than 11,000 bolts on 2,800 motors were replaced after a motor came loose and derailed in January, injuring 32 people.
BBC London's transport correspondent Andrew Winstanley said: "Quite simply if the bracket fails in service, the train would probably derail."
He added that half the fleet had been checked and so far 12 cases of cracked brackets had been found.
LU safety director Mike Strzelecki said: "We are confident Central Line trains are safe to operate.
"A small number of hairline cracks have been detected on Central Line trains, but we are now testing the whole fleet.
"We would not run these trains if we felt it was not safe to do so, and we have enhanced safety procedures for trains in service post-Chancery Lane."