Steps to reproduce
1. Open a game after 1957 (e.g. current B-B).
2. Open the Available Vehicles List (Ctrl+V in pak128.Britain-Ex)
3. Open the Train tab and filter by traction type diesels. Observe that it contains only electric vehicles.
(https://i.imgur.com/vfjl58H.png)
4. Filter by traction type steam. Observe that it contains only diesels.
(https://i.imgur.com/XsJLLsi.png)
5. Filter by traction type unknown. Observe that it contains only steam vehicles.
Notes
I suspect that an index value is off by one, but I have not looked at the code (yet).
I have incorporated a patch by Ranran that I think should fix this - I should be grateful if you could re-test with to-morrow's nightly build.