In scale model trains, the 2 rails used for running the train also power the train.
There are a variety of scales that use this method: O, HO, N, Z, G, and others. Some use a 3rd rail model or overhead lines either as an alternative method or to emulate the trains they are modelling. The brand Lionel typically uses a 3 rail track and is very similar to O scale trains in size.
These trains are often small enough to be run using low DC voltage, where direct skin contact between the rails gives either no, little, or minimal shock value. More advanced, newer trains use Digital Command Control (DCC) systems, this is an AC power system which acts as both the AC power source and has a digital signal embedded in it. The decoder in the train decodes the addressed commands and converts the AC to DC power for locomotive power.
Some larger scale models that can actually carry humans use a small gas, diesel, or oil engine. I'm sure you can use an electric motor, such as something from an electric car, so that's just a minor engineering detail. Getting more voltage and current to the motor through the track might be an issue, though, which is why larger trains don't put electricity though the load bearing wheels. I don't know all the specific details, but putting electricity through a bearing has issues.
I'm answering this way because just as scaling up something like this has drawbacks, so might scaling down another system. Using a 3rd rail or an overhead carriage might not work well for a smaller system, so it'd be easy to consider doing what I've mentioned, only to find out later that scaling it back up doesn't work as intended either. I haven't done research on this, so I don't know if there's a cut-off point or overlap where one system starts to fail and another system suddenly becomes useful. There may also be a speed factor, where a high speed method could be completely different from a low or medium speed method.
My intention here is to make you aware of a system not mentioned by others as well as letting you know that which type of system may depend entirely on what size or speed you want to make your project, so that you will have a good idea as to which system might work best for your intended use.
I'd suggest that you not only ask what systems currently exist, but also ask what worked for people in the size and speed range you are looking at. Asking that 2nd part of the question might end up being another question on SE, seeing as you already have the first part in this question. Linking or otherwise referencing the two questions would be a relevant way to ask that 2nd question.