This was exactly what I was going to say. It works....ask me how I know? :o

I agree, sew just a tad to the side of the line near the corner. Also the thicker the thread and bigger the needle the more space is taken up when folding so if you change needles to a different size or thread then you might have to make some other adjustments.
One way to check to see how much the fold is taking up is to press your triangle up to the corner before you trim the seam. The difference between the edge of the triangle and the original block is how much you're off and will give you an idea of how far to move the seam.

Once I know that my seam is correct, then I mark another line 1/2" from first one. Cut between 2 seams and you have a small HST to use in the border or something small.

I have always been told to sew on the inside of the railroad tires. So if you sew a little to the right of the line you should be good.