A vehicle has struck a bridge it was passing under in Tulse Hill, south London, causing train cancellations and delays of up to an hour.
Examiners attended the scene after the Volvo lorry hit the bridge, and have now ruled it safe for trains to travel over.
The line, which is used by operators Thameslink and Southern said the disruption, re-opened at around midday, but delays are expected until 3:45pm.
Trains running between Peckham Rye/Herne Hill and Streatham/Crystal Palace into London Bridge are being affected, but London buses are accepting tickets for the journey on "reasonable routes".
Eyewitnesses suggested the lorry was too big to pass under the low bridge.