In 2010 Oyster acceptance was expanded to include all commuter rail services within Greater London.
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"Based on witness statements and having reviewed the CCTV footage, there is no evidence of anyone spitting in this incident," said police.
Their cat Rothko, resplendent in a bow tie, was the only witness present.
Chad Gordon was shot in the head after answering his door in Haringey, north London last week in a suspected case of mistaken identity.