As someone who knows quite a lot about defined benefit pensions and risk transfer mechanisms, Abellio appears to have signed a very large blank cheque. Worse still, the Pensions Regulator is making it clear that if the govt is successful in offloading the future risk of historic pension liabilities (as they are obviously trying to do) then they will consider the deficit in that light. ie, with a less secure sponsoring employer(s), the deficit will grow and there will be a requirement to bring it up to an acceptable funding position quicker. =£££££. This could well dwarf any expected profit they expect to make.
So quite why Abellio has agreed to these terms is beyond me. Clearly nobody has signed up to them for South Eastern. So of the recent franchise competitions, XC was aborted as only Arriva was interested, and that was on the previous pension terms. South Western only had one compliant bid, which is now trying to renegotiate the terms as the reasons for Stagecoach making a non-compliant bid have turned out to be correct, East Midlands had one compliant bid (which was said to be the best of the three, yeah right!), South Eastern has none, and West Coast is down to two, and we don't even know whether they are compliant either. And the government says that that franchising is not in a mess.
If any challenge is unsuccessful then you can bet that Stagecoach and Arriva will be watching like a hawk to see whether there is any future renegotiation on the pension terms. I call that the TV-AM scenario, whereby they lost the breakfast franchise to a stupidly aggressive bid from GMTV, only for the latter to go back to the government a year or so into the franchise pleading (successfully) for the terms to be relaxed. TV-AM didn't exist by then, so couldn't challenge that was unfair, but Stagecoach will if it happens here.