I'm gonna take a stab at some of the ones you asked about specifically.

3GS-21B means that it has three gensets, which are individual engines. The 21 refers to a total of 2100 horsepower. The B refers to the axle configuration. A B-B locomotive has two trucks, each with two powered axles.

The RS11 was an older Alco locomotive. The RS stands for road switcher, 11 doesn't mean anything but that it was the 11th model (I think, Alco's numbering broke from sequential order at some point) of that particular type of locomotive.

SD70AC: EMD introduced the SD designation way back in the 60s, when it stood for a six axle "special duty" locomotive, as opposed to the four axle "general purpose" engines. The SD line has subsequently gone through many different models. The 70 dates from the 90s I think, when the original SD70 was built. That locomotive has since been refined into the most recent models. The AC refers to alternating current traction motors instead of direct current, which had been universally used until the 90s.

The P32AC-DM is a passenger locomotive, hence the "P". It has 3200 horsepower, indicated by the 32. It has AC traction motors. The DM refers to it being dual mode, powered by either a Diesel engine or an electric third rail.

I have very little idea about the H-24-66. I know it had 2400 horsepower, so that explains the 24. I don't know what the H indicates. The 66 maybe refers to the fact that, being a six axle locomotive, there are 6 wheel on each truck, but I don't know why they wouldn't use the standard C terminology if that's the case.

There is no standardized naming system in North America, it's all based on the manufacturers' decisions. That said, if you know what some common abbreviations mean you can often take a decent guess, especially on more modern units, which tend to have more complicated, but specific, model numbers.