The public might, someday, well beyond my lifetime working or otherwise, get a general transportation benefit from HSR or a future devolution of it. A connection between ARTIC-downtown-Disneyland, however, is primarily a private affair that has incidental effects on the public by, possibly, taking a few busloads of people out of traffic (if it's separated). Public funding, therefore, should be incidental (<<50%, more like <25%, imo) in the total package, if present at all.

Sounds like a job for Disneyland Monorail Route 2 to me. Note that in FL the monorail is in fact a system, with routes ocnnecting several parks and hotels within the complex to a central station where off-site and parking shuttle (bus) connections are made. While somewhat clunky, it can be used for more than just a sightseeing run around the park.