A little trick about Marble Madness:
In the Coin Op machine, when you play as a single player, you can use Both
trackballs at once.
Thus, you can move hella fast, as the extra input readings really add up.
It was probably to allow the player to use either left of right trackball, but players getting ticked off when they chose the wrong ball and wasted time... so programmers left both enabled the entire game.
Im not sure if mame emulates this feature of the game.