550 trains axed in 24-hour strike today (31st July).
Previous walkouts took place last Saturday and Thursday, and the remaining strikes are scheduled on 4, 11, 18, 31 August and 1 September.
In a statement, the rail company said it was "surprised" the union had decided to call further strikes, "especially at a time when families want to be out and about enjoying the summer holidays".
"We will continue to seek a resolution and work hard to minimise any disruption to our customers."
Ticket-holders get no compensation during train strikes.
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