It has been reported that a man spat popcorn at other passengers while on board a train from Euston to Tring on August 27 at around 6pm.
It is alleged that he then racially abused two passengers.
Officers met the train at Tring station and a man in his 30s was arrested on suspicion of racially aggravated harassment and threatening behaviour.
Anyone with information or witnesses are asked to get in touch by calling police on 0800 40 50 40 or texting 61016, quoting reference 514 of 27 August.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.
