i would definitely be willing to work on the 3d aspect and i always wanted a project to start with in C++, i know some C and other programing language used in certain game engines so i should be able to help. i would just want to know where you need to start and what needs doing.
Splendid!
For the programming side of things, a list of projects can be found here. If you are new to C++, I recommend starting with one of the simpler of them, especially as the codebase is a little old and not as modular as it might be (the original Simutrans, from which Simutrans-Extended derives, was written way back in 1997-1999 by somebody for whom this was his first C++ project, too).
For the 3d graphics side of things, there are two elements: the first is the research of the objects and the second is the creation of the 3d graphics themselves and the data (specified in text format files) to go along with them. The research for some things, such as buildings, is very straightforward (just find pictures of buildings in the relevant part of the world (for the "pakset" - that is, the set of graphics and data, that I maintain, that is the UK), build your model to the scale of the other buildings using the existing .blend files as templates, and fill in the data by extrapolating from the text files for other objects); for other things, such as historical vehicles, finding source material can be a lot harder, but I am able to assist with that aspect of things if you are able to produce the graphics.
Because the graphics are rendered in a low resolution before being used in the game, the 3d models need only be quite basic, which means that you can produce a good number in a short period of time (several per day once you get used to the workflow). It can be quite satisfying to see so many additional things being created for the game: I often wish that I had more time to spend on the graphics/research/data element of things, which I find more relaxing than the programming as there is much less scope for things to go wrong (errors are usually minor and easy to fix with the graphics/data).
I realise that I have so far explained this only at a very high level of generality. May I suggest that you register to join the International Simutrans Forum (Simutrans-Extended shares a forum and community with the original Simutrans, which is still actively developed itself), and have a look at this thread for information on how to get started making graphics for the pakset? (Although I should note that that thread is now a little out of date as the workflow has changed (and become easier) since the introduction of alpha blended sprites; but once you join the main forum, we can talk about the workflow in more detail, which is actually quite straightforward).
Thank you for your interest!