At first: I didn't check it in sandbox mode, but at the checkered flag scenario you can create a custom track for "River Rapids" coaster. You can start with some station rail and then go uphill with a 'gentle slope', but whatever you do; there is NO OPTION to get the rail back to flat! The 'gentle slope to flat' or simply 'flat' button is greyed out, no matter where you place the coaster or what kind rail there is before the 'gentle slope'. There is no option to create a 'normal' 'River Rapids' coaster with a simple slope uphill because the buttons you need are greyed out.
