BBC News Online takes a look at some images from the first morning of the action.

Normally busy platforms are empty at rush hour on Thursday

Thousands of commuters faced disruption

Management ran a skeleton service on peak arterial routes ...

... but many of Arriva's 1,600 daily services are cancelled

Members of the RMT union are demanding better pay

The RMT says guards' morale has been dented by "huge" pay increases for drivers

The union is threatening to hold another two-day strike next month

The company has apologised to its customers