A good point Cap’n D,
Most who try to compare the two lose sight of the fact that in Trainz, the physics is mainly about a single velocity vector. Everything else is constrained by the track. Trains to not move through a viscous and non-homogeneous medium that has a large effect on them.
Because of the very simple physics involved, and the fact that trains need only sit on track rather than be immersed in a medium with which they interact in a very complicated manner, adding rolling stock as individual items that can be downloaded and exist as separate file sets, is far more easy.
Pjotr has explained, very clearly, what is involved in adding ships, and why it will probably never be possible for users to create and add ships by themselves.
He has also offered a good alternative in a partnership between user and VSTEP. The principle drawback is that the user can’t tinker, but must be capable of doing a workmanlike job before submitting his model to VSTEP. However, anyone who is good or has the patience to become good can get the ship he wants.
Marty