World’s Easiest DIY Electric Train

The AAA batteries and C batteries have the same voltage, so they have the same force. However the C battery has more mass so it can't move as fast. Hence the smaller AAAA battery goes faster. Of course, the C battery has a larger capacity so it should run for longer before dying.

Running batteries in sequence increases the voltage so they have more force, but it's also not perfect because you are also increasing the mass. But if you found a battery with a larger voltage it should move faster.