Payment through PayPass will cost the same as having paid on Oyster - including the discounted rates. From 2013 that's £1.40 for a single ride. However, unlike using an Oyster Card, there are no daily limits. Cash payments will cost £2.40 from 2013.
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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.