Nick Mann, 35, was killed as he tried to save his brother Robert, 32, from an oncoming train at Old Street station.
His brother had jumped down to retrieve a wallet or phone, according to a report in the London Evening Standard.
The accident happened at 12.20am on 22 March. The brothers were taken to the Royal Free Hospital, but Nick Mann died later that day. His brother was still in intensive care at the time of publication.
Mann had previously been a reporter for a variety of trade websites including The Actuary, Public Finance and Letsrecycle but he had recently joined HMRC as a press officer.
Stephen Hardwick, HMRC's corporate communications director, said: "Everyone who knew Nick is shocked and numb. Nick was a very professional press officer and gifted writer, but more importantly, he was a genuinely nice man, friendly, full of fun and very popular with his colleagues. It is typical of Nick to have thought of his brother ahead of his own safety, and while we are devastated by his death we also feel proud of him too."