I've been using Energizer NIMH batteries for several years charged in an external charger. They last quite awhile. I've also found I can see well enough with the backlight turned off which greatly increases the battery life.
I really don't understand what Lionel was thinking when it comes to powering the Legacy handheld. The old TMCC handheld took 4 batteries and doesn't have a digitital display or backlight.
Why would they increase the power demand and shrink the supply?