Brick.ie The Irish Association for Adult Fans of LEGO®

Looking for some advice. Was attempting a narrow gauge train and was trying to keep everything as small as possible. Put the motor in one wagon and and managed to keep it 5.5 study wide. Made the gears so it went as slow as possible.The battery box and sensor went in another wagon. Though the wagon with the motor is too light, so sometimes the wheels will just turn and not move. Probably solution is to combine both in one unit to give it the weight it needs. Was just wondering would anyone have any other possible solutions or maybe I’m missing something basic? Thanks Snowy