Place a stone brick block down with a button on top, then an unpowered rail next to it, have 4 normal rails, then 3 powered rails, power them with a redstone torch, all rails should be in a line, then place as many rails as you would like after that and the mine cart will go on forever when a player is inside a mine cart will not go on forever if nothing is inside....tested in ,Creative, Survival, Nether, Overworld. Tested with creepers and skeletons and same effect. Also saving and quitting to title whilst open to LAN and in a mine cart the minecart will disappear, cheats or no cheats doesn't matter, just needs to be open to LAN.