British Transport Police have said the earlier closure of Streatham Station came after officers were called to reports of a knife fight.
A force spokewoman said: “Officers were called to reports of a fight involving a knife at Streatham Common Railway Station at 11:25am today (17 May).
“A man in his 50s have been arrested on suspicion of Actual Bodily Harm and Possession of a Bladed Article and remains in police custody.
“Two males in their 20s have been arrested for Affray. They were taken to hospital with injuries that were not thought to be life-threatening or life-changing. Both have now been discharged and remain in police custody.
“Anyone who witnessed the incident or has information is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40, quoting reference 202 of 17 May.”