1. I think it is only distance (euclidean distance, not actual distance travelled… so it is better to have straight than curvy lines )
2. I think people choose randomly four destination: 1 residential, 1 leisure, 1 work, 1 commercial and travel back and forth between residential and the other… e.g. R–>L–>R–>W–>R–>C–>… I’m not sure if after one cycle all destination LWC destination are redrawn.
3 No idea… can we put cars on trains ??? If you mean waggons, you can put as many as you want, but speed and acceleration decrease because of the added weight. In addition you should not have trains longer than station.
4. Alas you need to destroy the station !!! We asked developers many times to fix this, but they did not.
5. You can upgrade at a certain time with high speed lines, but if the limit depends on curvatures the only way you can upgrade your lines is by rebuilding them.