Sorry about the title. The problem is this:

Those are the accurate capacity values for my 4-extension train depot (unless that is a bug of its own)... because I just reloaded the world. That's the only time I'm aware of when the stored car capacities will update. I don't know if it's just a display glitch or if the actual capacity behind the scenes gets out of sync with the extensions.

I have also noticed that these capacities are almost completely ignored- allowing cars to go sometimes rather wildly over-capacity- when there's a train in active production.

Reproduction:

1. Have a train depot.
2. Have a train.
3. Scrap the train!
4. Add (or remove) a train depot extension.
5. Watch the capacity remain the same.
6. Save and reload the world. Game restart not required.
7. Notice that the capacity is now different than it was before.

Update 4.1 | v0.8.5 (b598)

Further experimentation has shown that this is purely stale capacities displayed in UI, the actual capacity behind the scenes is updated correctly. The "ignored when there's a train in active production" thing was a misconception, caused by forgetting that I'd expanded the depot hours earlier, but not reloaded the game between.