VTech Go! Go! bridge motorization Smart Wheels

My 2 year old kid, have already the Vtech mining theme. The train in this theme climbs the hill alone.
In the normal theme with the blue bridge you have to use a crank, therefor i designed the motorization.

Parts:
1x 12V Motor (esp. c9050-60003) old 2D printer motor (https://goo.gl/YMfPrH)
1x 4xAA Cell Holder (https://goo.gl/xLN2k8)
2x M3 Screws (17mm) M3 Nuts
2x M3 Screws (not longer then about 3mm) to screw the motor
1x 12V button/switch

I had cut a hole on this side, because there