The original note, as written at the time
SPACE HIGHWAYS, DRIVER BOOSTERS. In a future where we find the resources necessary to help us in space exploration, we might just find the idea of having booster gates arranged along our paths of exploration constructed beforehand. This will in turn help our crafts to be lighter and thus travel further.
Ongoing development
In a future where we have the resources for real space exploration, we might build booster gates arranged along our paths of exploration beforehand — so that our craft can be lighter, and therefore travel further.
The logic is the same as a railway rather than a car. If the energy infrastructure is on the route instead of in the vehicle, the vehicle carries no fuel for that stage, and every kilogram saved is either more payload or more range.
The gates would be fixed, enormous and few, and the craft would be small, cheap and many. That inversion is what makes the economics work: a mass driver on the moon costs a fortune once, while a fleet of spacecraft each carrying its own propellant costs a fortune per launch.
The scheduling problem is the real design constraint. A gate that fires at a fixed cadence forces traffic into windows, and a gate that fires on demand must store enormous energy between shots. Both are legitimate and they produce very different civilisations — one with timetables, one with queues.
And there is an obvious vulnerability that any fixed route has: the gates become the chokepoints, so whoever controls them controls the traffic. A highway in space is a political object before it is an engineering one.
The first version of this already exists in a modest form. Any propellant depot in orbit is a booster gate without the boost.
Build the road before the cars. It is the only way the cars get cheap.