The cab drivers are aware of when there's a train coming in and there'll usually be a few waiting, often minivans. If not you can call 714-999-9999 or 800-649-1222 to arrange for a
Yellow Cab.
The north-south train route is the Pacific Surfliner and always stops in both Anaheim and Fullerton (and Los Angeles, San Diego, Santa Barbara, etc). Someone coming in from Northern California would generally connect with the Surfliner in LA.
The east-west route is the Southwest Chief which starts in L.A., stops in Fullerton, and then heads east to Chicago. Doesn't stop in Anaheim.
Amtrak California is a good site:
http://www.amtrakcalifornia.com/index.cfm/destinations/disneyland/