St Pancras international platforms to Lille Europe
https://signal.eu.org/osm/#locs=51.530413,-0.125307;50.638752,3.075694 (make sure to select No High Speed Lines)
Blimey - are those sums right? (The link you gave showed distances and speeds, but not times
* - as far as I could find - and I didn't go through doing all the sums; and I'm surprised if StP-Lille could actually be done in 2.5 hours while avoiding all HS lines other than the tunnel itself. Especially since it wouldn't have those routes to itself, but would need to fit round other [stopping!] services...)
But would avoiding HS routes really save enough cost to enable a cheap-and-cheerful London-Lille at a
significantly lower price than existing services? If it really were a lot cheaper - and without booking months ahead - I guess there might be a market for it ... providing people were persuaded that the range of onward connections from Lille Europe was good enough.
Of course it would need tri-voltage stock (aren't French non-HS routes on a different system from the HS and international lines?).
But it's an option to expand international services from StP that I hadn't come across before. Even if the sums worked out, the range of onward connections from Lille might be a key factor in its attractiveness. Are there many services on to, say, German or Switzerland direct from Lille?
*Woops - sorry - later update. I see that site does give a total time (2h32m). Though presumably that's assuming running at full line speed on all of each section, with slick interconnections, and no other trains in the way. A 2.5 hour headline time might be attractive, but make it a more realistic 3 hours
or more, allowing for pathing, and unless it's
really cheap compared to Eurostar it might not be that attractive.
Maybe there's a compromise where it uses
some of the faster infrastructure, but still cuts costs in other ways - not that the basic service on Eurostar isn't fairly no-frills anyway. I suppose it's uncertain what trade-off of speed and price would work.
I'd see a big selling point as having walk-up fares close to the level for which Eurostar require booking many months ahead. The prohibitive expense of Eurostar is a lot to do with the need to book so far ahead to make it affordable. So a cheap and cheerful walk-up service would to good business I'm sure - though whether it would be viable without having a fair few high-priced seats to cover costs I don't know. If there were
walk-up tickets, even without being any cheaper than the lowest tier of Eurostar prices, I'd
happily put up with 3 or more hours on the slow lines to Lille!