The select committee is taking a close interest in progress of the PSU because the project is the result of what transport minister Kim Howells once called 'a massive clanger'.
The PSU is a vital part of plans to take all southern region slam door trains out of service. The project, the UK's largest ever fleet renewal, came out of the official report into the 1988 Clapham rail disaster which concluded that the structural integrity of the old Mark 1 trains was not good enough.
With no discernible progress after 10 years, legislation was passed in 1999 to force the fleet renewal through, and a deadline of the end of this year imposed. This is about to be extended to the end of 2005.
It was only when orders had been placed for half the 2,025 new train vehicles that it was realised that they used significantly more power than the old Mark 1's and the network could not support them.
'The new trains are approximately 25% heavier than the old ones, ' says Mitchell.
'And they have air conditioning and electric doors.
As a result they draw 23% more power than the Mark 1's and they have a different power profile.'
Railtrack, now Network Rail, to get more power to the third rail - cue the PSU project.