As a graduate of one of America’s elite law faculties and a sworn adherent of New York’s subway system, Hillary Clinton should have had a firmer grasp of the network’s byzantine rules on electioneering.

The Democratic presidential frontrunner broke the subway’s regulations on political campaigning when she boarded a busy train in Manhattan on Thursday and made no attempt at feigning anonymity.

Instead, she happily colluded when Ruben Diaz Jr, the president of Bronx Borough who was accompanying her, introduced her to startled passengers as “the next president of the United States, Hillary Clinton” - following up by chatting up fellow travellers, who were perhaps more receptive than normal in the run up to this month's New York primary election.

That broke Section 1050.6 (c)1 of the subway rule book, the New York Post reported, which forbids campaigning in trains but permits it in stations as long as long as candidates stay at least 25 feet away from ticket machines and booths.

For breaking that code, Mrs Clinton could potentially face a US$25 (£17.70) fine or a 10-day jail term - although it remains to be seen whether New York’s Metropolitan Transportation Authority will enforce the law.

Mrs Clinton, who has a degree from Yale Law School, has faced more pressing legal problems. An FBI investigation into her use of a private email server while she was US secretary of state is continuing. She is also the only first lady to have been subpoenaed to provide evidence in an inquiry into the Whitewater affair during her husband’s presidency.