Staff thought a man who boarded a train at Manchester Piccadilly was sleeping - only to realise he was dead at the end of the five-hour, 250 mile journey.
The 'elderly' passenger boarded a Cross Country service to Bournemouth on Saturday, May 30.
Staff thought he was sleeping when they passed him during the journey.
When the train arrived at Bournemouth station, workers realised the man had not left.
Emergency services were called and the man was pronounced dead.
It is not known at what point in the journey the passenger died.
British Transport Police (BTP) said the man's death was caused by a 'medical matter'.
His family has been informed.
Cross Country said that during the coronavirus pandemic, crews have been walking through trains, but, due to social distancing, 'contact' with passengers has been reduced.
Since the start of lockdown, people have been advised only to use transport for essential travel - or if they're a key worker.
A spokesperson for British Transport Police said: "We were called at 2.41pm to Bournemouth station on May 30 following reports of a sudden death on board a train.
"The death was determined to be a medical matter, and the person’s family have been informed."
A CrossCountry spokesman said: "The elderly gentleman had been seen by the train crew during the journey and thought to be sleeping, but when they did not depart when the journey ended the emergency services were called who confirmed the customer had sadly passed away.
"Our onboard teams continue to walk through trains as part of their duties, helping customers on their journeys where they can.
"However, their interaction with our customers is currently reduced to maintain the recommended distance from others we are all asked to observe."
