Go to this link: https://conradlempert.github.io/mini-metro-map-maker/

Here is the source code: https://github.com/conradlempert/mini-metro-map-maker

First, create a picture that you want to use as a background while drawing shapes. This could be a Google Maps image or a metro map.


Next, open this image in the tool and adjust the settings. 5000x5000 was recommended by the official guide. You can desaturate the image so you can see your shapes better.


Now, you can draw points or polygons. When you do this, their coordinates get automatically copied into the clipboard. Points are exported like [x,y] and polygons are exported like [[x1, y1], [x2, y2], [x3, y3], ...]. This format can be directly used in the city.json.


Here is further explanation of the tool:


This is what the final result looks like:


Please note that the upload/download functionality is only for shapes from this tool. This is not directly compatible with the .json files in a mod folder. You have to copy/paste the coordinates.