korell: Not sure I follow you then as I don't remember trying to use a ride whilst I was building it. I do remember some rides giving error messages if they had incomplete track, but that was dependent on the type of ride, so those ones might not allow you to open the ride at all until it is finished.
tinyE: When you've completed the ride (it has to be finished or you get the error message like you say) and then click on the flag it gives you a drop down menu with 'test' and 'open'. 'Test' is suppose to run just with the cars, 'Open' with passengers. In build it allows you to open the ride but no one gets on.
Oh wait, is this the RCT2 coaster construction box (i.e. not the actual park, just the coaster in a gridded area)? In which case, I won't know the answer as I only played within the parks. I wouldn't have thought you'd get people on it but it would make sense to be able to provided the weight of passengers had an effect on the ride itself. Did RCT physics ever include passenger weight though?