Ahh, this comes up every so often, and sounds like a great idea on the face of it, but it is not very practical.
Bottom line, the effort/cost to make it really work well is just not worth it.
First of all, there are INDEED high current transfer wireless systems, but they are too expensive for us, you cannot get the loops close enough to make it work with any reasonable cost. If you have put a thin plastic case on a cell phone with a wireless charger you know this already.
So, you would need to do contacts to charge the battery. This works, and is often used on those layouts with the little cars that run around and you don't have metal rails.
The next obstacle is one that is mentioned, you MUST have the charger in the loco, and to be worthwhile, it must be a fast and intelligent charger.... That can be done, but now you are adding extra cost, probably close to the cost and bulk of the battery.
You need a charger that can correctly "restart" from power interruptions.
So, wireless, no, with contacts yes, but you will incur a lot of extra space and cost penalties to accomplish. Bigger batteries would be a more reasonable alternative, or batteries that can be quickly swapped out.
Remember that even with a fast charger, you still have to wait a bit, and also you would choose battery packs that would tolerate C or higher charging rates (C is amp hours)
A bit more on the charging rate: a 5000 milliamp hour battery has a "C" of 5 amp hours. Fast charging to be useful is multiples of C, so even just 2C is a 10 amp rate, now your contacts need to be weatherproof and handle pretty darn high current, another maintenance nightmare.
Greg