Connecting multiple carriages together to form a train was an amazing innovation - those carriages are separated by inches, whereas trains are separated by miles.
If everything could be separated by inches, we could have a network with 1000x the throughput. 1000x the throughput means we can use all the existing (expensive) rails, and be able to transport everything the interstate road network does today.
And, with enough innovation, that's possible. Imagine an infinitely long train. If more cargo needed to be added to the train, while in motion at say 60 mph, one link in the train could be broken, the gap widened to fit an extra carriage, and then an extra carriage comes in from a side rail, just like merging on a highway, and the whole lot is reattached again. All while still moving.
Infinitely long trains already exist - for example an escalator is a continuous 'train' of steps in a loop.
Disconnecting trains while in motion already exists - it was done regularly on the british railway 50 years ago.
And the rest is just a coordination and timing problem - one that computers are really good at.