Available Vehicles List does not correctly filter vehicles by traction type

[Matthew]

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.

4. Filter by traction type steam. Observe that it contains only diesels.

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).

[jamespetts]

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.