Gearing problem?

Hello everybody, I have made a locomotive some time ago, but it has some problems now since recently, I think with the gearing, so that is why I post it here. When running my locomotive forward it runs slower and less smooth than backwards. I thought the wheels were the problem, because they wobble a little bit, but I took them off and it was still there. The wheels slow down at a certain point, so I thought maybe the gears are rubbing against something, but I took out all the gears and put...