[gaazoh]

Acceleration and deceleration aren't relevant here, the limit would be passenger comfort either way.

Curved tracks do mean limits on top speed, as the centrifugal force needs to be kept below what would be uncomfortable for passengers. It also can cause some issues for building stations, as curved stations require a wider gap between the train and the platform. But neither is a blocking issue, all train tracks in the world do have curve, and curved stations aren't unusual in public transport systems.

The grandparent probably means disk, not ring-shaped when mentioning circles anyway. Pick any 2 point at random into a 33km² circle, the average distance will be 4km, and worst case scenario 6km. Do the same thing in a 33km², 200m-wide "line", the average distance will be 85km, with a worst case of 170km. A circular city doesn't need nearly as much raw speed for it's public transport to be more efficient than transport in a nonsensical linear city could ever be, even if you throw in ridiculously fast trains and sprinkle magical AI thinking.

It's not a coincidence that all major cities are roughly circular even though they are built around roughly linear features (navigable rivers and/or coast line). It's just what naturally works.