Basically, minecart momentum appears to be capped, even though there is no reason it should be capped.

To recreate: Build a 30 block tall rise with powered rails (powered on), then build a 60 block long downwards slope. Then, create a 40 meter upwards slope afterwards. Both of the latter slopes are with normal rails. Then, ride a minecart across the rollercoaster. You should only make it to 22 blocks up the final slope before going back down. (Check F3 to confirm)

If this isn't a bug, then why is it part of the game? Also, I know this is intentional, but why is minecart speed capped at 8 m/s?