And solve the trivial engineering task of reducing all friction and air resistance to zero.

Well shit, anyone can do that. Just put a little WD40 on it

You build your track dead straight - like, not conforming to the surface direct through the crust straight. Now the train accelerates downhill for the first half of the journey, and decelerates uphill for the second, neatly coming to a stop at the destination. Oddly enough, in the spherical cow universe where you build this, all the maths cancels such that you get a constant travel time regardless of the start and end locations. On earth it's about 40 minutes

You need energy to decelerate, though.

🤔 A vacuum tube maglev would do the trick.

Actually Isaac Arthur talks about something like that on his channel. An Orbital Ring, he calls it.

you could conceivably get on and off the train with shuttle "station" trains that travel on parallel tracks to catch up with the main train