I have to wonder why no one's come out with a system to keep the batteries charged from track power. You'd still get the non-flickering, but not have to worry about the batteries dieing.
I have, in the early 1990. I had a small NiCad battery charged from the tracks powering 1.5V bulbs which illuminated interior of an observation car. That was back in the dark ages (DC powered models).
I'm sure others have also come up with a similar circuit. Nowadays (DCC), all one needs is a keep-alive capacitor. Unlike rechargeable batteries (which have limited life and often leak), keep-alive capacitors can be recharged almost unlimited time and they will remain sealed for decades (no leaks). Richmond Controls makes and sells such a circuit. I have also designed a similar circuit (using cheaper smaller value tantalum or electrolytic caps). One of TRW members is currently using it.