It's a nice bright and fun quilt for a child who is attracted to trains.
To me ... my first audition would be to follow the same style as already used with the smaller blocks.
That is, black then a narrow colour strip then black again.
I would try for the blacks to be slightly wider than you have used for the smaller blocks..
Perhaps the 1st outside black, would be the same width as the 2nd of the smaller blocks.
Then the 2nd outside black, the same proportion wider as with the two blacks in the smaller blocks.
For the colour strip, I would audition, both the light green of your background and a scrappy mix of all the other border colours.
Good Luck!