This is a code I made to have previews for my custom coasters.
Requirements:
RCT 1 (only tested on the original CD-ROM version of the game, + expansions)
Python (3.0+?); packages:
  matplotlib
  Pillow
Basic instructions
1) Save a custom ride in-game
2) Place the ride in a big open area (I use an Arid Heights park)
3) Remove base land and vertical faces so there's a black background; test the ride
3b) If your ride uses black, you might want to re-color it so it shows up better.
4) Get a good snapshot; it's okay if there's stuff around the edges.
5) Run the program. I think it's set up to just run.
6) Select the snapshot file in the program folder, which will be a deprecated image filetype.
7) Pyplot with show you the image. Take note of the coordinates of where you'd like to crop the image.
8) Enter to coordinates as directed.
9) The program should allow you to save the resulting image.
10) Name the file the same as your custom ride. Move it to the Custom Rides folder in the program directory.
11) Open the game and test it out! Let me know if you have issues.