> The widths are fixed depending on the line > there isn't usually more than three car variants using any given line I'm not sure about the first point, do yo

> The widths are fixed depending on the line > there isn't usually more than three car variants using any given line I'm not sure about the first point, do you have a source? As for the second point, sure — because there are currently 3 different door layouts on the A system and 3 on the B system. But those layouts are all freely mixed across lines right now. Even if they were not, interlining means trains of different lengths will be stopping at the same platform, which means incompatibility. I suggest skimming the MTA report linked in the article, it goes into quite a lot of detail on this, including diagrams.