It would if the Underground were directly accessible from the high speed platforms by lifts and escalators, and without any ticket gate delays. But that's not and never was the plan. Instead people will have to exit via the existing concourse and walk outside to the Underground entrance. Some people will take over 15 minutes to walk from the far end of a 400 metre platform to the current Underground entrance.They absolutely could. It'd involve not timing the Euston end quite as slackly as it is now, but it's not impossible.