I looked at Google Maps and the tracks that serve the "second-best" boathouse location continue along the riverfront into the park lands between downtown Cincinnati and the river. While its not ideal to have active tracks through a park
[Link], somebody thought them practical enough for passenger service that the
Riverfront Transit Center (built in 2003 before the 2010 assessment for the Ohio 3C plan) was intended to be a commuter rail terminal. The
commuter rail plans were still floating around as recently as 2016.
Also, I took a second look at the environmental assessment and:
(1) the line(s) serving CUT were ruled out due to freight congestion (hat-tip to
Eric S)
(2) the boathouse location had broad support but intense opposition.
(3) Riverfront Transit Center isn't listed as even a considered alternative for the Cincinnati station.