A passenger rushing to catch a train at Ealing Broadway was headbutted after asking a man who was blocking his way to 'hurry up'.
The victim was changing platforms and was suddenly attacked on a footbridge.
The attack happened on Wednesday, 11 March during the evening rush hour.
A 40-year-old man, from Southall, was stood on platform three waiting for a Reading-bound train to arrive. Shortly before it was due, passengers were informed of a platform alteration via a station announcement.
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting T/B6 of 27/05/2015.