The energy density on super capacitors is pretty bad. If you imagine full power 200kW braking for 5 seconds that's 1 mega joule and at a best case 8 watt hours per liter you're going to need 35 liters minimum. Really you probably need to double that so you can float up and down and never fully saturate the capacitor as power inflow is going to drop as you get closer and closer to fully charged.
https://en.wikipedia.org/wiki/Supercapacitor