Good Morning JonMrylennBailey
Are they appearing with snow on them, or just without leaves?
Trainz supports both 'winter' and 'snow' settings for seasonal assets; these are very much separate things when talking about 'generic' content. If the snow-line setting in your environment settings is too low, then seasonal assets placed above the specified altitude will have snow on them.
If it's just a lack of leaves, then maybe try setting the date to around January and see if you see the 'correct' look. This may mean that your world origin is set incorrectly. The controls for the world origin are on the 3rd tab of the environment settings (same location as the toggle for turning traffic on/off).
Note, all seasonal assets should be setup pretty much the same for the 'date' of the seasons. Trainz will then handle when those seasons will occur (for northern or southern hemisphere locations).
Regards