It’s an indirect power that comes from Disney’s control because Disney doesn’t have to compete with other constituents. If the City of Orlando was short staffed Universal could lobby for more staff but could ultimately not prevail.
I would guess that Universal’s recently proposed community development district is all about gaining some control and special treatment from SunRail. While many are still focused on Brightline, SunRail offering transit for $2 - $3 per person is probably the bigger win for Universal. SunRail though is a rather Spartans service: stations are just open air platforms, ticketing is by self service kiosks, station staffing is minimal to non-existent and service is not consistent or offered on weekends. The new CDD could use its transit authority to build a station with amenities, subsidize service, contract for more frequent service, provide bus services through the district, etc. all to the clear advantage of Universal. Depending on the final boundaries of the district, Universal’s large land holding could definitely make them a major player in how this new district operates.