This issue appears to have been fixed and so this page is now archived.
According to TfL the Oyster day runs from 0430 to 0429 the following day. This was chosen because no tube or DLR services were running at that time in the morning. Yes, there are night buses, but as you only touch in on a bus and the reader is only connected to the Oyster system when back in the depot, there was never a problem with them. However, several National Rail lines do run throughout the night, or at least a lot later than the tube. Since National Rail joined the Oyster system a number of people have experienced problems when travelling in the early hours. They manifest themselves either as a pair of incomplete journeys, or a journey being charged after the daily price cap had been reached.
Eventually I have managed to find a copy of a reply from TfL on the Rail-UK forum:
Please be assured that the issue you have described is known to Transport for London’s (TfL) fares and ticketing department who are working on a solution. However, the problem in its current state is essentially an unavoidable consequence of the extension of Oyster pay as you go to National Rail (NR). We have done checks and found that it is extremely rare that customers are affected by it. When customers use National Rail in the small hours, the Oyster daily price capping arrangements occasionally don’t work as intended due to the need to spread the times when the end of day/start of day processing takes place at individual stations. Thankfully we know that most customers usingOyster pay as you go on National Rail overnight are out of the scope of price capping anyway.
When the Oyster system moves over from one day to the next for charging purposes, the processing demands on the system are very heavy and to keep the system operational it is necessary to avoid having one single time point for all stations to change over. The processes have always been spread out for Underground stations, but with no impact on customers because the Underground does not operate at the hours concerned. Just occasionally customers travelling overnight with National Rail will find that the system fails to identify that their journey belongs to its correct day for capping purposes.
Having checked your journey history it has been confirmed that you were charged £4.60 more than would have been if price caps had been applied as advertised. In order for me to arrange the most suitable medium through which you can receive the refund of the above amount, please contact me direct on ***********
Please be assured that the issue is being monitored and accept that if it turns out that significant numbers of customers are being affected, we will need to make arrangements for publicising that overnight NR journeys may fall outside the arrangements for daily price capping.
I hope the information I have provided has been helpful. Feel free to contact me should you have any future queries.
Transport for London
I think that this is actually a pretty serious error. Whilst I understand that they need to stagger the end-of-day processing, surely they could arrange for the few stations with all night services to do it at nearer to the 0430 advertised time.