Faster minecarts when multiple linked as train

First, single minecart mechanics remain the same. When multiple carts are coupled (say, with a chain), then they can go faster. Eg, 1 cart stays at 8m/s, 2 carts for 9 m/s, 3 at 10m/s, to a maximum of say 16 carts at 24m/s.

Obviously, longer trains should have more momentum, but it is harder to go uphills and around corners. Eg, a 5 long train should only go uphill at 1 in 5 (ie, go along 5 blocks before going up 1 block) and corners should be at 5 block radius circles (ie, not sharp turns).

To help control these, perhaps make the speed powered rail boosts too dependent on  redstone signal strength. So, if rail is powered at strength 3, then a minecart will speed up/slow down to 6m/s. Detector rails could also sense speed and output at a strength.