This incident took place on board the 8.10pm Leeds to Malton service on Saturday November 8.
A 47-year-old man from the Scarborough area was arrested at Malton station in connection with the incident.
He was interviewed and has been released on police bail pending further enquiries until November 30.
Investigating officer, PC Nick Storey, said: “A man became aggressive and confrontational with other members of the public on board the service, which culminated in him spitting at another passenger.
“Police were called and met the service at Malton, and the man was subsequently arrested.
“We would like to hear from anyone who may have been travelling on the 8.10pm Leeds to Malton service last Saturday night and witnessed the incident. If that was you, please contact us.”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016. Alternatively, you can call the independent charity Crimestoppers, anonymously, on 0800 555 111.
In all calls, quote incident reference PSUB/B5 of 14/11/14