PC Frances Tilley admitted she had "destroyed her career" after evading payment for a journey between Bristol Temple Meads station and Castle Cary in Somerset last June.
A disciplinary hearing at Northamptonshire Police headquarters heard she was less than two years into her job and was still on probation.
She was dismissed for gross misconduct.
The hearing at Wootton Hall heard Tilley travelled from Northampton to Castle Cary on 22 June last year.
The officer, described as "diligent, professional and committed", was travelling with friends and did not buy a ticket for the final leg of her journey.
Force solicitor Elizabeth Briggs said: "She travelled from Bristol to Westbury and onto Castle Cary. At that stage, she did not have a valid ticket to travel.
"There were opportunities for her to purchase a ticket.
"She said it was her intention to not pay a rail fare for that part of the journey."
A single off-peak ticket from Bristol Temple Meads to Castle Cary would cost £15.60 if bought today.
The exact cost of the journey at the time of the offence is not known.   Tilley went on to pay a fine of £109.65.
The hearing was told that when challenged, the officer claimed she had taken a shorter journey and was "rude and abrupt" to a revenue inspector.
Tilley sobbed as she told the hearing: "I have worked for years doing my university degree and I have just destroyed it.
"I let myself down, if I could change anything I would."
Dismissing her, Northamptonshire Chief Constable Simon Edens said: "It's simply unacceptable that a police officer cannot be trusted because of our unique role in society and we must be relied upon to tell the truth."
A police officer has been sacked for failing to pay for a rail ticket that would have cost about £15.50.