I'm not keen on long fixed formation units, much prefer the flexibility offered by shorter fixed forms with through connection coupled up to create longer units - 4 car units (24/25m car length) on TPE NW would give great flexibility, giving 4, 8 or 12 carriages, but admittedly wouldn't work for TPE North/South, which would probably need to stick to 3/6 car formations. Plus surely it's not beyond the genius of man to predict passenger numbers based on ticket sales using a few algorithms to look at previous travel history of travellers with flexible tickets.