(You will need a mushroom for this shortcut.)
Choose Yoshi's Circuit as your track. To cut across Yoshi's spine, when you come out of the second tunnel, a little bit out you will see a space in between the rail and the rest of the track. Now use your mushroom, and speed yourself off the ledge. If your timing was right, you should get ahead by alot!
(P.S If you miss, say hello to a watery grave!)