The original note, as written at the time
IMPROVED TERRESTRIAL RAIL LAUNCHER. To overcome the challenge of constructing a long enough rail gun or EM driver capable of launching vehicles using electromagnetic induction into space from earth, and to overcome the challenge of the over-the-surface support towers that could be needed for such an undertaking, the system would start its rail from deep under the ground below sea level; it would then climb up through the ground.
Ongoing development
An improved terrestrial rail launcher: to overcome the problem of building a rail gun long enough to launch a vehicle into space, and the problem of the support towers such a structure would need, the rail begins deep underground, below sea level, and climbs through the ground.
Burying the launch track is a genuinely good idea and it solves two problems at once. The structure is supported by the earth rather than by towers, so the cost per metre falls dramatically. And a launch happens underground, where the noise and the blast are contained, which is what has kept rail launchers off every populated coast.
Starting below sea level is the clever detail. It gives you a long run-up in solid rock, it gives you a place to put the machinery, and it means the exit can be a shallow angle rather than a vertical shot — which is the difference between a launch and a gun.
The physical limits do not move. A rail launch puts enormous acceleration on the payload, so it can carry cargo and not people; and the atmosphere is still there at the exit, so the vehicle needs a shield and a way to survive the transition.
The honest use is bulk cargo — water, fuel, structural material, the things that make a space programme possible and that currently cost more to lift than to make. A launcher that moves tonnes of water is worth more than one that moves a person.
Dig first. The ground is the cheapest structure we have.