The Crime and the Criminal

The "crime" in question initially starts out being about a man named Tennant getting on a train, having a row with a woman in the carriage and her "falling out of the train". He fails to act (this is a prevalent trope in Marsh's work - the characters in question end up becoming criminals due to the failure to act when necessary) and it turns out that the woman is dead. Or is she? (Goodreads)