Thank you for your email and I am sorry for all the delays this week. On Monday morning our 08:06 service from Oxford was held up by Network Rail improvement works between Swindon & Didcot that finished much later than planned. This had a significant impact on our ability to operate a punctual service and your train, along with many others (88 in total) suffered a delay as a result.
The delay you experienced on Tuesday stemmed from a signal problem. I completely understand that lots of small delays stack up and are equally as frustrating. Fifteen hours of delays since we started corresponding is of course significant.
We too expect better and Network Rail, who manage and maintain the UK infrastructure (i.e the track, signals and engineering works), are very well aware of this. I mentioned in a previous email that they had been issued with a warning by the ORR, and we too are applying pressure for improvements to be made.
On Wednesday our 08:06 was delayed at Didcot whilst we took action to fix a train which had a broken window. On Friday, there was a temporary speed restriction in place in the Banbury area, and so our 08:06 got caught behind those ahead that had to run more slowly.
The reason why trains are so susceptible to congestion, is simply because of the number of trains in service, particularly at peak times. There is very little room for movement because of train pathing constraints. If one thing happens in one place, even a one minute delay can knock the timetable out of sync. We do have contingency plans in place, without which these delays would be much more significant, but with the track at capacity, congestion can't be eradicated completely. That said, the work undertaken to duel the Cotswolds line has helped, and the remodelling of Reading station will make a significant difference when complete.