I enjoy doing things by hand sometimes. Working with good old hand tools keeps my hands honest.
To strengthen the rail tenon I drilled a 1/4" hole just past the mortise I just made.
Then I cut some 1/4" round aluminium just a little larger than the height of the tenon.
And pounded it through the tenon with a hammer.
All I have to do now is file the aluminium flush and do some finish sanding to get things ready for the finish.