I'm not sure RCT ever had a modding community, but it's predecessor, Transport Tycoon Deluxe, did. TTDPatch[0] had several gameplay and quality of life improvements, but it was eventually superseded by OpenTTD[1].

[0] https://www.ttdpatch.net/

[1] https://www.openttd.org/

If anyone’s curious to see what a lossless 6016x3384 screenshot of OpenTDD would look like:

https://dmitri.shuralyov.com/temp/6K/OpenTTD.png (25.6 MB)

There is OpenRCT2: https://openrct2.io/