Where can I learn more about gearing and torque with Power Functions motors?

[Carefree_Dude]

When building a MOC steam engine the other day, I discovered the gearing I had done allowed for the train to move itself, but not pull any carriages. This cause me to have to redo a bunch of stuff. So, I was wondering... 

How can I learn how much torque and RPMs each of the PF motors has? (M, L, XL etc)
What is the ideal torque for a steam engine? 
How does multiple motors work when trying to account for what is needed? 
How can I learn more about gearing? I know gearing down increases torque and lowers rpm, and the opposite lowers torque and increases rpm, but is there some good math to go by, and what gears do what with each other?
Say a train has 4 drive wheels, should all four be geared, or do only two of them need to be like the emerald night? Is it better to gear all the drive wheels? 
 

[skaako]

I always use Philo's page for getting the details I need about the electric motors. It has a ton of information on every detail you could imagine.

http://www.philohome.com/motors/motorcomp.htm

Mike

[Duq]

Indeed, Philo's page is the goto place for any Lego electric specs.

For torque, if you're totally new to engineering I would start with a look here, Khan academy explain a lot of things really well:

https://www.khanacademy.org/science/physics/torque-angular-momentum/torque-tutorial/a/torque

There are no hard and fast rules for what torque you need for a steam engine, or any train for that matter, or which motors or how many. It all depends on what sort of train you are building.

If you want to pull long trains slowly, then you'll need lots of torque. Gear down the motor to make your train stronger but slower. If you want to build a high speed train then gear up for more speed, but it may take a while to get to that speed because of the lower torque.

For grip or traction it's best to have lots of powered wheels as it reduces the chance of wheelspin.

For a steam engine it depends on how you make your side rods. If they are rigid, like liftarms or half-beams, then you only need to power one axle because all the wheels are connected by the side rods. However I found that if you make your side rods with axles and connectors there's a little play in the length of the side rod. In that case it's better to connect the axles with gears.

Hope that helps. Looking forward to pictures of your trains.