Hundreds of commuters were removed from a train service after guards spotted "sexually offensive" graffiti on the side of a carriage.
The Thameslink service from St Albans to Wimbledon was terminated at Blackfriars to avoid causing "further offence" to passengers.
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The franchise's owner, Govia Thameslink Railway, also runs Southern, which has recently been at the heart of an industrial dispute with staff.
The "unusual" decision to halt a train service that was fully functional has been blasted by commuters.
Speaking to the Evening Standard, one passengers, Laura Bufton, 31, said it was a "joke", adding: ""I couldn't make out what it said and I wasn't offended by it at all. This is completely crazy. Why cancel a perfectly working train and inconvenience so many people on the off-chance someone might be offended?
"I'm sure people would prefer to get to work with offensive slogans on a train than have it cancelled."
A spokesman for Thameslink told the Daily Mail: "We are sorry passengers were inconvenienced but we simply cannot have our trains running with offensive sexually explicit words on them."