For assets (graphics, sound, etc.), yeah. OpenTTD, an older sibling project, eventually was blessed with artists willing to remake all of the assets so the game could be played without the original TTD installed.

I think a lot of the charm of RCT2 comes from the specific assets in use and the degree of whimsy and joy they contain, more so than TTD's matter-of-fact trains and planes and such.

It's not like these folks are setting out to make a totally free-as-in-beer game that replaces the old one; they just want to be able to fix bugs and add features, and you can't easily do that with the original retail stuff.

RCT2 is now 20 years old - what do we as a society gain by still not letting those assets be freely redistributed and built upon.

Take it up with Disney, not these fine folk.

https://en.m.wikipedia.org/wiki/Copyright_Term_Extension_Act

Those are commercially available from gog.com: https://www.gog.com/en/game/rollercoaster_tycoon_2

And the data files from the demo version do the trick too.

The way they have been working on this has been interesting. Rather than a from scratch approach, they started with the original game and over time replaced component by component the original game bits with open source replacements.

That sounds like that wouldn't hold up in court, it's not a "white room" implementation by any means. I hope it won't be tested though.

Contrast that to OpenTTD or OpenRA where they (AFAIK) re-implemented the engine while still having the same approach of re-using the main games assets.

Edit: taking a look at the source code, I don't think what you're saying is actually true. RollerCoaster Tycoon 2 was written entirely in Assembly but I cannot find any Assembly files in OpenRCT2. So unless they already finished porting everything, it seems incorrect.

They have finished porting everything. And it was done chunk-by-chunk, but in C++ instead of assembly.

I looked at the source a few years back and saw more than a few functions that looked like the result of decompiling machine code to C. I didn't spend much time trying to see if that's true or not, but in the end it doesn't seem to matter. Still, if you like the game, you should keep a local fork of the repository, just in case.

I think this is the go-to approach for most of these kind of projects. You decompile it and then start refactoring it into C-functions and such that make sense. That this game was originally written in assembly probably helped a lot making the disassemby readable.

I had the discs years ago but lost them at some point. They're available online to pirate, but GOG also has them for a few dollars.

You can use the demo files for RCT2 to play OpenRCT2 for free. It's not encouraged, but OpenRCT2s website has a download link for it.

That's very common for projects like this. ScummVM is another example. If you don't require the original files to run, you're essentially redistributing a commercial game for free, which even for older titles is generally not advised.