It's worth noting that the report reaches no conclusion at all - it doesn't aim to, it's just documenting what happened. The most interesting paragraph (for readers here) is I think 4.6, about "Delivering benefits without electrification: Cardiff to Swansea":
4.6 The main passenger benefits of electrifying the Great Western route were to increase passenger capacity and to improve passenger experience. The Department had already ordered new bi-mode trains to run on the route before the decision to cancel. The train specification includes 24% more seats than the trains they replace. The Department expects to achieve the majority of journey time benefits through running the bi-mode trains in electric mode between London and Cardiff, where the trains can run at 125 miles per hour where line speed allows. The maximum speed a train (diesel or electric) can run between Cardiff and Swansea is 90 miles per hour so the full benefits of running fast electric trains would not have been possible even with electrification. Hitachi, the train manufacturer, estimates that electrification will reduce journey times between London and Swansea by 15 minutes once electrification to Cardiff is completed in 2019. The Department’s appraisal of the project in April 2017 assumed that journey times between Cardiff and Swansea will be 3.5 minutes longer with bi-mode trains operating in diesel rather than electric mode. However, at the Department’s request, the manufacturer has agreed to modify the bi-mode trains to enable them to operate at faster speeds in diesel mode. The Department currently expects that once electrification to Cardiff has been completed, the trains will be able to achieve the same journey times in diesel mode between Cardiff and Swansea as they would have in electric mode.
