Who’s talking TGV? I was talking maglev in a vacuumed environment, absolutely no limit to length relative to speed. As to car width given they would be building system from scratch can make the cars to whatever dimensions are desired.
This is not something that is based on 40 year old tgv, at least not how I see it.
Nonvacuumed maglev would cost, at the absolute lowest price $20mm per km just for rail - or $225B, not including the price of terraforming, building a tunnel, an accompanying grid, or any of the rolling stock.
If it cost as much as the proposed Japanese projects, instead of the pie-in-the-sky low number that Chinese engineers are predicted, it would cost $3T.