She was sitting by an open carriage window when the man crept along the platform and tipped a bottle over her head, The Sun reported.
British Transport Police have released CCTV footage of a man they are hunting following the disgusting assault at Plumstead train station, south London on the morning of May 24.
Investigating officer PC Tracy Mesher said: "The unsuspecting victim had taken a seat beneath an open window of the 20.46 Plumstead to Greenwich service and was waiting for the train to depart.
"She felt a liquid run from her head down onto her clothes. The horrified woman recognised the smell to be urine. She realised that the liquid had come from a drinks bottle which had been placed through the window."