Train too slow using rechargeable batteries

As above, the rechargeable batteries have lower voltages than standard batteries. It may also be worth checking if your rechargeable battery is a new one? Rechargeable batteries have a finite life and their performance can deteriorate over time. So is probably your rechargeable! Try a different one and see if it improves any, AAA batteries have a small capacity anyway, it wouldn't have to loose much performance to stop it being suitable for use in something like this, although it might be still ok for use in a tv remote as it doesn't use as much current.