The Spanish High Speed line through a limestone aquifer was abandoned when the water poured in
A key failing of HS2 was that it bypassed the regular project safeguards that most railway work has to meet - namely the GRIP stages 1 to 8. These mirror what is referred to as a stage-gate process, where the preceding stage need to be complete before passing the gateway to the next stage
Thus the project scope and needs have to be assessed before evaluating route & other options
One moment that really delivered a spat out my coffee moment was when in 2008 a claim was made that no new main line had been built in over 130 years
Thanks to a visionary MD of the railway that crossed under the Pennines on a near direct line between Manchester(Liverpool) & Hull, who then bought into the Metropolitan Railway and the SECR, and in 1896 planned the main line to get from Sheffield to Aylesbury, and then to a terminus at Marylebone, but also connecting through via SECR to a Channel Tunnel near Dover. In 1906 (with the twisting and busy Metropolitan Railway a big constraint) the direct main line with parliamentary powers to build 4 tracks, high speed alignments, and provision to enlarge clearances for European mainland trains we had a joint GC/GW railway to Sheffield (right up the middle) and Birmingham
It was a good railway and the prototype Class 47 averaged 100mph for 7 miles (105mph through Bicester) on jointed track with semaphore signals in 1962. However in 1968 the line from Euston to Birmingham was electrified, and to make the figures look good the GC line was closed to a residual backwater to force all passengers on to the Euston route
There are moves that can salvage what damage has happened to date but there are many ways to get capacity through a programme of much smaller connected and separately delivered details, very much more like the EGIP programme in Scotland
For the headline routes Glasgow-and Edinburgh-London, each side has around 3 major 'slacks'
ECML - Morpeth (50mph) Newcastle (20mph) and York (35mph)
2 - new lines bypass tight bends (Morpeth - inland - York from Clifton to Askham under Holkgate Hill)
Biggest challenge - Newcastle - ideally a tunnel Heaton to Tyne Yard
WCML - Carlisle (20mph) Preston (40mph?) and Stafford/Crewe (various)
Other less severe ones apply near level crossings (74 were in a plan in 2013 that's made minimal progress between London and Doncaster), and non-stop runs of under 4 hours have been done to Euston and Kings Cross with the current trains
4 hours centre to centre is a cleaner and less piecemeal journey than flying, with the ability to do work (earning fees rather than raising costs, plus also having meetings with people joining the train for part of the trip (I used to do debriefing of Grimshaw after a visit to Scotland on the 1 hour trip Glasgow-Carlisle with wine, cheese & rolls from Peckhams)
Remember too that neither LNER or Avanti are using IntercityRailfreight with cycle couriers doing final mile and premium parcels service - London to Scotland door to door in 5 hours or less
Also noting daily 100mph freight train Birmingham-Motherwell does round trip in around 8 hours replacing up to 24 trucks & drivers, 6-8 hours one way at 56mph top speed
Al lot to push for using the existing network plus taking back the old lines kept intact by cycle routes, with a deal that was almost delivered as 'agreed' by Network Rail to build a parallel cycle route, which would be opened when the railway was opened - cue for pestering MSP's about the failure to deliver the section of cycle route Plains to Drumgelloch.