Even though battery-powered locomotives are somewhat new, the garden-gauge world has had them for a while, but the components they use are far too large to be used in the smaller, indoor modeling scales. The deciding factor is the amount of current the locomotive needs to do all of its tasks versus the size of the electronic components needed to handle such current. G-scale locomotives measure their power draw in many amps, while the smaller scales, such as S-, HO-, and N-scale measure theirs in terms of milli-amps. Older S-scale (such as American Flyer) and O-scale engines kind of fall in between, and their needs will typically need to be weighed on a case-by-case basis. Sometimes the solution is to replace the motor with a modern, more efficient one before installing the battery-powered system of choice; sometimes a more powerful decoder is necessary.
Here, I wish to capture some basic information about the various control systems that are currently available within which a battery-powered solution can be used. They are described in alphabetical order. The Stanton S-CAB system will be described separately, since that is what I use. Please use this article only as a starter. I would highly recommend that you visit each of the manufacturers' web site and dig deeply into whatever documentation you can find on their system, before making a decision as to which one you would prefer to use.
There is an article in the March 2017 issue of the Model Railroad Hobbyist magazine, starting on page 18, where the author covers the CVP, Tam Valley Depot, and S-CAB systems. Only the first one he has any real experience with (due to his garden railroading experience). However, in my view, the article is fairly accurate.
This company makes components that allow you to control your locomotive via Bluetooth communication (typically using your smart phone or tablet). Because that system doesn't use the rails to communicate information, having it powered by a battery within the locomotive is a distinct possibility. It requires a custom solution, and it is my understanding that the battery cannot be charged via the rails, unless one makes a custom installation. The system is not DCC-compliant, but it does support consisting of up to 4 locomotives. It supports two-way communication.
(external link: BlueRail Trains)
Their web site's "User Showcase" page shows various installations done by people who have bought their system, including one for S-scale.
Bob Walker has written some articles about installing this system in O-scale locomotives, namely the LCCA The Lion Roars (Oct. 2014, Feb. 2015, June 2015, Oct. 2015, Dec. 2015, Oct. 2017), and O Gauge Railroading (Dec. 2016, Apr. 2018) magazine issues. The User Showcase link also provides a link to a YouTube video by Bob showing some of the engines in action.
(external link: Darby Marriott SHS SW9 Consisted)
CVP Products makes a system for the large-scale community, called the AirWire system. For the smaller scales they introduced the microAirWire system in 2016, which can output up to 1.5 amps (5A surge). All their decoders are DCC-compliant. They have two throttles that can be used to control all of these AirWire systems. The T5000 PRO throttle is also a complete and self-contained decoder programmer (see the throttle on the left in the photo above). I do not see where their system would allow charging from the track (a feature not needed in G-gauge railroading), so you may need to provide an externally-accessible plug to hook up to an appropriate battery charger, or be able to remove the battery from the locomotive when it is time to recharge. One thing to note here is that their T5000 PRO is compatible with the S-CAB system, so you can use the CVP throttle to run and program your S-CAB decoder.
(external link: CVP Products)
Dead Rail Installs
Tam Valley Depot created a solution with a bit of a different approach. It presumes that you have an existing DCC-based system, takes that system's existing DCC signal, broadcasts it into the air, which is then received by the locomotive, where both the engine and the receiver/decoder can, optionally, be powered by battery. You control your engine via your existing DCC system's throttle. This eliminates the dirty track issue, but it does require that you have an existing DCC system. The batteries are lithium-polymer, and the system doesn't support charging the battery from within the engine from the track (battery removal, or external plug is required). If you are starting from scratch, this is an expensive solution, because you have to first buy a DCC system and then buy their components and batteries. In October 2017 Tam Valley Depot announces that they will no longer be producing their dead-rail system. Sometime later, however, Dead Rail Installs announced that they have taken over the transmitters and receivers of the Tam Valley Depot system. This company is also an authorized installer for BlueRail Trains, CVP Products, and Soundtraxx systems.
(external link: Dead Rail Installs)
DelTang is a more conventional radio-control (R/C) solution, based in England, which may be more appropriate for European customers. It seems to work for smaller scales, but it does seem to require a bit more electronics knowledge to install the system. It doesn't directly support sound decoders, and is not DCC-compliant. Remember that with radio-broadcast signals, the manufacturers have to have their systems certified for use within a country. So be sure to check that for the system in which you are interested. It is my understanding that the DelTang system can be used in the U.S., but it cannot be sold by a dealer in the U.S. (you would have to buy it directly from a British dealer and have him/her send it to you in the U.S.). My impression from studying their web site is that the system requires quite a bit of tinkering, and does not support charging the batteries from the track. You must provide your own batteries and an appropriate charger. Contact fellow S-scale modeler Bob South, if you want to seriously pursue the DelTang system (he can help with advice and custom installations of the DelTang systems and any other brand of battery power radio-control system in plastic, metal, and brass locomotives). Miles Hale did a TrainMasters.tv show (subscription-based), where he showed how he installed a DelTang chip in a small On3 engine.
(external link: DelTang)
The LocoFi system can support DC between 9 and 24 volts, which, according to their web site, qualifies it to run on a battery-powered system that can output that kind of voltage. They don't provide batteries, so you are on your own with that. I have gotten an e-mail from a fellow S-scale modeler who is using the LocoFi system with the S-CAB BPS module to allow him to run this engines from his smartphone and not rely on track power.
(external link: LocoFi)
Micro Flier Radio
Micro Flier Radio is company that makes R/C components for very small flying model airplanes. Several S-scale modelers have adapted this system, along with some of the Stanton's S-CAB system components to build a custom solution for controlling trains. This definitely requires a lot of custom and creative work. Since their products are not aimed at model railroaders, their control system might be an issue.
(external link: Micro Flier Radio)
This British company provides a battery-powered solution that uses a radio-frequency controller. It looks like an interesting system that is designed for OO-scale, which means it might work in S-scale and some smaller O-scale locomotives in the U.S. It requires the installation of a plug board, so that an external charger can be hooked up to charge the internal battery. It does offer an interesting way of turning on the battery power. Supports two-way communication.
(external link: Protocab)
The RailPro system, by Ring Engineering, uses radio-frequency control, but their system doesn't provide battery power. There is a YouTube video of someone who converted an HO-scale engine to battery power with the RailPro system (note his disclaimer in the YouTube comments section). If you are looking for only a radio-frequency-based system, this is another option to consider (as well as all other regular DCC systems that offer wireless throttles, such as Digitrax, NCE, etc.). It supports consisting.
(external link: RailPro)