I think it's going to take future historians to unravel who benefited the most from the deliberate, politically expedient delay of a project that, if it had been a highway with the same "shovel ready" status and jobs profile, would have been approved forthwith and likely given federal funding under the 2009 stimulus bill.
As you say, rail operators have benefited, though the incremental benefit attributable to KXL, which as I understand it would only carry around 100,000 bbl/day of oil from the Bakken shale, pales compared to that from the other 900,000 bbl/day they'd be carrying either way. Rail is expensive, but its flexibility suits the moment. Transcanada's competitors in the pipeline business have also arguably benefited, as have investors in those companies.
Environmental groups have also clearly benefited--aside from the benefits they would claim the planet is receiving due to the delay--because KXL has been a bonanza for them from an organizing and contributions standpoint. One big piece of pipe makes a better symbol than thousands of individual wells, for example.
If Keystone is to be approved in the next 2 years, I'd guess it will only happen as part of a deal with even bigger symbolic value to its opponents.