Are you thinking of Network Rail's 2007 "Preliminary Evaluation of Double Deck & Extra Long Train Operations" report?
... Paul
Well, that must be the 2007 report - which was evidently published, but by the government, and spirited away into the archive soon after. But I don't think I've seen it before. Probably. So thanks for that, it does show a lot of the issues.
Of course it also shows the assumptions made, many of which could be challenged by a bit of imagination.