which all brings me back to one of my original points... why not do the work to convert to DD when infrastructure projects are taking place anyway? all the work being done on GWR for example to electrify the line... including I assume rebuilding over bridges to accomodate wires.... what would have the extra cost have been to rebuild to accomodate DD's so that they can be operated in the future if so desired?
When new bridges over the railway are constructed, they are almost always built to GB+ gauge. Same goes for overbridge reconstruction, unless the practicalities and therefore cost of realigning approach roads etc is difficult. (For example, if you can rebuild as is without land take, but you need land take to go a higher gauge, then you stick with it as it is to avoid several years delay.) Tunnels are a different matter, I can only think of only one tunnel that have been completely rebuilt in the past quarter of a century (as opposed to simply regauged) and I don’t know whether it, Farnworth, was rebuilt to GB+. Plenty of other have been regauged, almost all by dropping the invert (some have been done twice, e.g. Ipswich) but it gets progressively more expensive by unit of depth the deeper you go.
To go back to the report; I happen to know one of the authors. Sensible chap. It was not written with the objective of dismissing double deck at all, although that was the effective result.
To re-answer the original question - why is double deck not considered at all: it is because this study, the Wessex study and plenty of other unpublished back of envelope studies have shown that the cost and in particular the disruption to existing services of regauging the whole network, is simply much greater than alternatives, which include building entirely new lines (see HS1, HS2, Crossrail 1, and dare I say Crossrail 2 and NPR, etc.). The new lines can also offer new connections and quicker journeyed, which double deck can not.
