Richard Lyddon, 36, from Taunton, Somerset, died from multiple stab wounds after the incident at Reading station, Berkshire, in January last year.
Thomas Stewart, 42, of Reading, denies murder. He claims he was drunk and carrying the knife in order to sell it at work.
He told Reading Crown Court: 'I didn't intend anybody any harm that day. I acted stupidly, foolishly, recklessly. I was in quite a reasonably merry mood when I entered the toilets. What started off as a rather farcical situation turned into something horrific and terrible.'
The jury was told earlier that Mr Stewart told police: 'I can't remember what happened but the knife and the scabbard became separated. It came out and hit the ground. I can't even remember what the man looked like, but he must have turned and thought I was coming after him.'
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