Both Rollercoaster Tycoon and Transport Tycoon Deluxe (which lives on in openTTD) are by the same author, and use the same engine :-)

Chris Sawyer, he made them with a tiny magnet that he used to write the games directly to a hard drive.

I once heard he tickled a butterfly in Brazil to cause a cloud in the UK that diverted a cosmic ray onto his hard drive in the exact right spot to flip the required bit.

The biome button graphics are taken from the OpenTTD main menu.