Why would Liverpool justify only one train? Are you assuming all trains will be 400m and double-decked? Because if so, I struggle to see Manchester or Leeds filling three of them per hour.
Maybe it will not be possible to fill three an hour, every hour, during the day at Manchester and Leeds but anticipated peak demand warrants that level of service; these cities are very large regional hubs, and rail connections into them will be available and will be improved from a wide hinterland. Liverpool, simply because it is situated on the coast, serves a smaller hinterland so demand is always going to be limited to an extent. If Liverpool captive trains were not double deck or full length, the only advantage you get over classic compatible trains over the classic line for the last 35 miles of a 180 mile journey is a very marginally faster journey time. The capacity of a 200m single deck captive train will be no greater than the same length classic compatible set, and running one of these alone could be seen as poor use of a trunk London path.
In continental Europe high speed trains are all 'classic compatible' with their historic networks because there is no effective difference in loading gauge, and the trains routinely run to a wide number of destinations away from the high speed trunks without expensive dedicated infrastructure stretching into every urban area. Our UK classic compatibles will allow a similar approach and really must stop being seen as somehow a second best. On the high speed trunk itself they will achieve speed equal to the captive trains and they will have similar seating capacity for a given length to a captive spec single decker, both at about 70% of a double-decker of the same length.
Currently an hourly 11 car pendolino provides around 600 seats per hour.
Two 200m single deck HS sets per hour, whether classic compatible or captive will provide around 1000 seats per hour.
Two 200m double deck HS sets, or one 400m double deck HS set per hour will provide around 1400 seats per hour
Two 400m double deckers per hour would provide 2800 seats per hour.
Clearly that suggests an upgrade path. An initial 400 seats an hour improvement can be provided with classic compatibles on classic route. If demand growth justifies it in the future a GC gauge cleared corridor, whether entirely new or not, could be engineered to allow double deck operation. That could be improved progressively to allow full length train operation when demand justified it.
It is likely that any 200m HS sets running alone at the extremes of, or beyond, the HS2 network, whether they are captive or classic compatible, double or single deck, will be combined at some major hub with another such service for the run over the trunk section from Birmingham into London, to make best use of the paths and the platforms at Euston.
