Plarail 'Doki Doki'

For the use of the phrase 'doki doki' (どきどき) in relation to 'Roller Coaster Mountain', see here:
https://www.blueplastictracks.com/showth...3#pid44993

Here's another box that uses 'doki doki' (the sound of a beating heart), released a couple of months later in 2007.

Percy and the Doki Doki Freight Wagons:

 

Here the phrase is presumably referring to the rattling-freight wagons, though you'll notice that there's an insert on the box showing Percy descending doki doki mountain as well.