Since 14w26c, or possibly earlier, but after 14w21b, mob-minecart-behavious seems to have changed, making it impossible to eject mobs in some situations.
The problem seems to be that mobs (tested with zombies) immediately reenter the minecart when ejected, which can often not even been seen by clients, to them it looks like the ejection fails completely. On a server with higher tick lag it is clearly visible.
This doesn't always happen, it seems to be directional: The problem occurs when the railline is west-east and the only possible exit is south (when driving in either direction).
To clarify, this worked in previous versions and broke in (probably) the latest snapshot (14w26a).
Edit: Affects 1.8.1-pre4