Police have released a CCTV image of a man they want to trace following the incident at Dewsbury rail station.
A British Transport Police spokesman said: "A man spat at a member of staff and became extremely aggressive after she challenged him for ticket evasion at around 2.25pm on May 11. The spit landed on the victims clothing.
"Do you recognise the man in this image? We’d like to speak to him as it is believed he may have information which could help our investigation."
Anyone with information is asked to contact British Tansport Police by calling 0800 40 50 40 or texting 61016, with reference 237 of May 14.
Or call Crimestoppers anonymously on 0800 555 111.