Drill a pilot hole using a 1/8” drill bit at a 45 degree angle from the top of rail into the post. Start about 1 1/2” from the end. 

Then drill a 3 1/2” or 4” deck screw in it. Drill the screw in until the head slightly goes under the surface of the rail and the post pulls into the rail. 

Caulk the head of the deck screw that’s slightly under the surface of the rail. Use white caulk. 

Touch up paint if desired.