Forgive my ignorance but how are the international platforms at St Pancras accessed from anywhere other than HS1??? Can you get to them easily from either the ECML or the MML or does it involve a bit of shunting or ground frames etc etc???
According to Trackmaps, there is a direct electrified connection (single track) from the international platforms towards the ECML and WCML via the North London Incline.
The eastward connection to the ECML is called, reasonably enough, the ECML Connection, while the westward one towards the NLL/WCML is called the Silo Curve.
There is also a single track connection to the MML but it is unelectrified and called the Maintenance Siding, so hardly main line.
As far as I can see, these connections have been never been used by passenger trains.