One is Windermere Park on Lake Wash. It's got a pretty serious gate at Ambleside which you can see from street view . Apparently they lock the gate at 7 PM each night and check membership during the day. That gate definitely looks hoppable, but another entry option would be to kayak up from UW. Windermere Park is labelled on Google Maps which is a bit misleading as it's not a public park.
Perhaps even more appealing is the Blue Ridge Private Beach . I guess Blue Ridge used to be a private community before it was annexed by Seattle, and it still has some of that left over; there are still homeowner rules and membership fees. The park is not labelled on maps, but you can clearly see the path to the beach in satellite view. The fencing around the park looks like pretty serious barbed wire, so the best entry option seems to be walking the train tracks up from Golden Gardens or down from Carkeek.
BTW, I've never understood how people die on train tracks or why there are so many signs about how "dangerous" it is. All around Carkeek there are these signs to stay off the tracks and that the trains are "fast and quiet". Uhh... I'm pretty sure if one of our trains surprises you, you must be absolutely drunk out of your mind, like almost unconscious. I mean, I understand with like a bullet train, those are in fact fast and quiet (though they also run on very straight tracks, so you can see them coming for miles), but our trains go maybe 60 mph and rattle and screech like motherfuckers. You could be a deaf giant sloth and still have no trouble avoiding a train. The signs should really say "warning : don't get black-out drunk on train tracks". Or the sign should just have a handgun on a chain and say "please kill yourself here so you don't derail the train".