It's harder to build a grid of straight railways. You need more bridging or crossing at the very least. You are right about 2D being more efficient in terms of

It's harder to build a grid of straight railways. You need more bridging or crossing at the very least. You are right about 2D being more efficient in terms of the length between two random points, but there could be scenarios where the simplicity still outweighs that benefit, especially given all the other factors in this project, like sunlight. The length is also much less of a concern than in traditional transport systems because of less friction and less need to brake, so most of the energy is for accelerating and decelerating at the stops. Of course it would blow up if you wanted to build a fully connected city that is twice that length or even longer, but that's not proposed.