Firstly it is extremely unlikely that the problem has been caused by moving from Windows 10 to 11. I have not seen any reports in the forums of Windows 11 causing this problem. If anything Trainz runs slightly better in 11 than it does in 10.
Secondly, the label "Obsolete" does not mean "faulty". It simply means that you have a newer version of the asset already installed thus obsoleting the original asset. Downloading that specific obsolete asset again will not change that fact that a newer version is already installed.
I will assume that you have reinstalled the same version of Trainz onto the new computer. The problem, in my view, must be caused by a difference between the two installs. The trick is to narrow down those differences.
Have you installed on the new computer all the DLC routes, sessions, etc that you had installed on the old computer?