The Eurostar terminal within the St Pancras train-shed cannot also function as the London terminal for the coming UK high-speed rail system – because there just isn't space for extra platforms. But the railway lands to the north aren't the answer – even if space could be found, it would be left isolated, remote from the Tube hub at King's Cross/St Pancras.
The optimum site for a domestic high-speed terminal is Euston. It's only 600m from St Pancras, perhaps five minutes away by dedicated underground travelator, and it's soon to be redeveloped.
Euston also has the largest ground plan of all the north London termini, and the necessary extra capacity could be created simply by diverting its commuter traffic onto Crossrail. This would address the ludicrous imbalance in current Crossrail proposals, whereby nearly half of the 24 trains per hour on the central section will terminate at Paddington. Commuters would benefit too, no longer forced to disembark at Euston and crowd onto the Northern and Victoria Lines.
COLIN ELLIFF (M), email@example.com