Who picks the trains?

Just curious... gonna use S&S woodies for an example. Who chooses what trains will be used for the ride? Do the parks have to find trains, or does S&S tell them they can only use Gerstlauer or PTC? Just wondering because the Denmark woodie is using Gerstlauer, while Clementon is using PTC.

I think the parks have the option on which they want.

Chuck, who says unlike CCI who couldn't offer PTC's due to faliure to pay unless the parks purchased them themselves.