I disagree that it won't impact tourism to central Florida for several reasons.
This train wouldn't just connect MCO to the parks... is connects Miami, to Palm Beach, to MCO, to the Parks, and eventually Tampa. This means residents of all of those locations will be more likely to visit each of those connecting cities... which means more Universal/Disney visitors.
And news of the train will spread to tourists worldwide. I've often heard of the excellent public transit over in Western Europe and Japan, just as an example, which makes my visitation to those locations more likely. Having good public transportation draws in tourists, due to the ease of getting around for relatively cheap (you can do/see more for less money). Thinking about them saving money on a taxi ride from MCO to the parks is small scale... The big picture here is that tourists right now have no easy way of getting from Central Florida to the beaches. Having a train going directly to Tampa/West Palm/Miami is a huge draw for the state. (It could also increase return visits for those who had a good experience with the train.)
It's really not even a question of whether it increases attendance (it will). The real question is by how much. It's not going to double attendance over night, but even a 1% attendance increase is pretty significant.