Do you just find the mine cart level annoying, or just don't feel like playing it? When you reach Mine Cart Madness, do not jump into the barrel that shoots you into the cart, but rather jump over it and curve back around the wall, and you'll be shot over half the level. It's useful if you really don't like the level.
At the screen where you select which game you would like to load, press these buttons: B, A, R, R, A, L and you should hear a noise. Once you enter the game, you'll notice that you'll have 50 extra lives.
P.S. This is kinda wierd, but if you play the very first level of the game, jump on top of the cave that exits the level, and run to the top of it. Just hold right on the D-pad and jump over and over, and if you do it right you should get 5 free lives. If it doesn't work, sorry, you're doing something wrong.