What motivated the companies to overbid? I don't see how that was potentially advantageous to them.
It must be very difficult to work out precisely the real cost of running a franchise. There are some things you cannot estimate - you just have to allow contingencies. You can make an estimate of passenger growth, but forecasting what will actually happen is impossible. Major external factors may impinge on what you'd planned. If your bid is cautious, someone may outbid you. It costs a lot to make a bid: I think at towards the end of the franchising period, it was frequently around £2 million. You don't want to spend all that and lose. Anyway, what is "overbid"? You won't know till someone's done it.
Some examples of the uncertainties in all this:
1) According to Roger Ford in about 2007, bids two and three for the South Western franchise totalled less than the bid from Stagecoach. I think the figures very roughly were Bids 2 and 3 about £650,000 million each and Stagecoach £1.3 billion. Stagecoach ran the franchise successfully at that level, but they could have won with £700,000 million.
2) In 2007/08, National Express bid more for the East Coast than GNER had a few years previously, but GNER had failed. Why did NatEx do that? They had been losing franchises and may have been desperate to win something. Could they have run it successfully? We don't know, because the 2008 recession came along and passenger numbers dropped.
3) West Coast in 2012: it is widely considered that First had overbid and ought to thank Richard Branson for seeking a judicial review which DfT found they didn't have the evidence to contest, so the offer to First was withdrawn. We'll never know how things would have panned out.
4) South Western in 2017: First/MTR won. One version of the story says they bid less than Stagecoach, but Stagecoach hadn't fully met the specification DfT had issued. First/MTR were making losses before Covid arrived and changed everything. We'll never know if their planned timetable and frequencies would have worked; doubts certainly exist.
Is there a common factor? Yes, the DfT, which
ought to be able to assess whether bids are realistic and achievable. But their crystal ball isn't perfect either, and no doubt they were under pressure to get the best possible price for each franchise in order to justify the amounts that the Treasury were putting into the railways.