Can someone check my math here? That link has some actual specs on their flywheel–a 1m diameter disc weighing about 500 kg, designed to rotate at up to 2500 rpm. I calculate that out as storing about 2 million joules of energy at maximum speed. That may sound like a big number–but a liter of diesel stores over 38 million joules … Did I drop a decimal point somewhere? Even with increased efficiencies, I don’t see 50 ml of gas taking your tram up a hill.