I have just had a similar situation on the buses. I didn't have enough money on my oyster to pay for the bus fare and was issued with a court summons. I called the prosecutions department at tfl and explained that it was an honest mistake, I was a student and that a criminal record would affect my chances of getting a job in the future due to one mistake. I was then told that if I paid an out of court settlement fee of £102, my case would be dropped.