Nice train.
Possible brick build "thin red line" solution (assuming the line is needed just under the gray and yellow rounded slopes) :
-Move the black plates.
-Use the thin side of any of these brackets underneath the edge the roof section:
http://www.bricklink...tem.asp?P=44728
http://www.bricklink...tem.asp?P=99207
http://www.bricklink...tem.asp?P=93274
All avalible in red and lbgrey. Two in yellow, One in light gray
Try the 99207 brackets with a plate placed on the studs on "thick part" of the brackets. Should lock the roof in place. There seems to be enough clearence above the batterybox for this solution. Maybe the front part of the roof needs to be detached.