Let me tell you, this is really obnoxious.

Best way I can think of to fix this is to disallow unloading from all Locomotives, unless the filter is set to a Locomotive. Basically an opt-in feature.

Or to allow carts to decide.

The PR just disables draining, but I'm pretty sure draining was enabled at some point because people playing with finite water wanted it.

So @purpleposeidon, Adding a filter to the unloader is the ultimate solution.