You Trainz content still exists or should.
By default, Trainz content is stored in C:/Users\<login> /App Data/Local/N3V Games/trs22/build followed by an alph-numeric string. Unless Steam users have their data stored elsewhere.
Note: trs22 can be replaced with trs19 or TANE.
When you start up Trainz...
At the Launcher, click on Trainz settings
Click on Install tab.
Click on the 3 dots to the right of the path located in the white box.
Browse to the C:/users/<login>/App Data location mentioned above.
You should see a path with Build and the alpha-numeric string. If there's more than one, chose the one with the older date.
Click on Select.
Trainz will want to restart and it may want to run a database repair.
Let it update and if all goes well, you should be all set since your username and login are already there.