Battery technology is rapidly advancing, there are a bunch of improvements to battery technology that are well beyond the LiPo technology that are being developed that will increase the power /weight of batteries and also decrease the charging times. Take a look at cordless tools and the difference these days is staggering, they have replaced a lot of corded tools even among pros, because they can do the job. Run out of juice? Swap out the battery pack while the current one charges. With cars, the distance they can go is longer and longer and charging times have dropped, there are cars coming on the market that aren't exotics that may have a range of 300 miles and be able to get an 80% charge in 15 minutes (there also are claims of batteries that can go 300 miles and be swapped out in 90 seconds, would be kind of like swapping a blue Rhino propane tank, though I put that with a 200 mph carb back in the 1970s, until it actually hits the market).
I haven't looked at the RC world, I don't know if the batteries they have there now are better than they were 10 years ago, but the answer to that may be that they don't have much of a need for higher density batteries, or the people who provide them don't have any reason to change, not the first time if that is true. Before Tesla came out with his car, the US auto industry was claiming electric cars were not practical, that they would be nothing more than a golf cart, until suddenly someone showed they were either deluded or outright lying *shrug*). FYI Tesla is no longer an expensive play toy, they are selling 500,000 cars a year in the US, that is not a small amount.
And if the battery is easily swapped out, might make running trains more realistic, if you had a battery the size of an SD memory card you plug into a slot on the engine or tender, if an engine's battery is running down, means you would have to pull the train over, swap the battery with a charged one, and go one, simulating a water stop on a steam engine or perhaps a planned stop.
The real question is will the makers of trains switch over to battery technology? That is a million dollar question, there is no reason you can't integrate battery technology with control technology like DCS, DCC or Legacy or Lionchief, but it depends on whether they see a market for it and it is worth switching, will they sell enough to justify the cost of switching power, and I don't have an answer for it. It doesn't mean powered rails will go away, some will be happy the same way they are with conventional control today, but it could eventually be where engines will be dual power the way they are conventional/cc today.
