There are a lot of tests which need to be undertaken, especially for trains which were not designed specifically with the tunnel in mind (as the Eurostar fleet were). However, if all goes well, Deutsche Bahn are expecting to run a service from London to Frankfurt, via Brussels and Cologne, from the end of 2013. The service is is expected to take 4-5 hours between the two financial capitals, about the same time as London to Edinburgh; while conventional wisdom says that rail is not competitive against air travel for journeys longer than four hours, this may no longer be the case, thanks partly to longer air check-ins and tighter security restrictions, and partly to Deutsche Bahn's exceedingly comfortable trains, or so Mark Smith (of The Man In Seat 61 claims):
A direct train could cut London-Cologne to 3 hrs 55 mins. This would compete with air not only on speed and convenience, but on comfort – DB's ICE trains are among the most comfortable trains in the world, being designed to tempt German businessmen out of their BMWs and Mercedes, with power sockets for laptops at every seat and WiFi on many routes. And using DB's current ICE fares to neighbouring countries as a guide, I'd expect a London-Cologne or London-Frankfurt journey on any new service to start at a very affordable €49 (£41) or even €39 each way, with no need to pay to get add the cost of getting to and from airports.I'm not convinced it'd be that cheap; the tunnel, after all, is a privately-run monopoly, with steep access fees, which would be factored into the ticket prices, though I imagine that it might well end up cannibalising the air travel market between London and Frankfurt (at least for scheduled flights; high-powered businessmen with private jets would presumably keep those), much as Eurostar did to air routes between London and Paris and Brussels.