A Tomy Bulk Buy

And I'll just add that you often don't even need to remove the motors from the train. Just stick a good battery in, flip it to the on position and give it some taps. This way you'll know as soon as it kicks over. The tender-powered engines can be trickier because unless you want to remove the front chasis that houses the motor, you are limited to just giving it an open handed smack (of course you need to be careful that you don't jostle it hard enough to risk severing the wires connecting the tender).