Transport for London (TfL) bosses faced questions today over Tube performance after members of the London Assembly said there had been a rise in delays on the network of late, identifying staffing issues as a growing cause of the problem.
In both of the last two periods, to 31 March, lost customer hours on the Underground - which is used as a measure of network reliability - surpassed the 2m mark. That reflects some of the worst performance levels posted in a year.
Lost customer hours are defined by TfL as those which have arisen from all service disruption incidents lasting two minutes or more, due to all causes, including those outside London Underground's direct control, but excluding the impact from planned works.
Services on three lines make up over half of all long delays. According to TfL figures, the District, Piccadilly and Central lines between them accounted for 55 per cent of all delays over 15 minutes.
TfL data which the London Assembly Transport Committee has been examining in a meeting today, points to four causes contributing to the most Tube delays. These were customer problems such as passengers taken ill or blocking the doors, problems with the fleet, signalling problems, and staff shortages.
Over the past four years, issues with customers and signals have stayed fairly level, while fleet problems were more of a trouble in 2016 and 2017.
Staffing issues however, have contributed to an increasing number of lost customer hours in the past two years, and these are not due to strikes as they are reported separately. They relate to issues such as staff absence and shortages.
Nigel Holness, director of network operations at TfL, told the London Assembly Transport Committee today the transport body was "trying to be as efficient as possible".
He acknowledged that there had been "one or two decisions about the level of resourcing" which had been reflected in the number of delays caused by availability of staff last summer.
What I would say is that we're in the best position now that we've been in for two years for availability and staff-related errors, so I like to think we've come out of that.
The Piccadilly Line was cited as an example as a "feeder line", getting "a huge amount of turnover" and new drivers coming in for a period before moving onto other lines. Holness said TfL had to make sure it gets the pipeline right.
As for rising delays in the past few months, Holness said that was "a blip" but many of those were occurring on the oldest part of the network, which was undergoing work to improve it.
"The future is bright, but the journey onto that destination can be quite painful at times," he said.