Steam Locomotive wear should be proportional to speed and 1.1 X driver diameter = max speed with minimal wear

There are other factors that would affect the rate of part wear or cause additional part wear, which could be a combination of global and per locomotive/car configurations. The effects could range from minor to catastrophic.

Some examples:

Mechanical:
-Most mechanical components in general should have wear, with more wear if the part is operating at a faster speed or under a greater load.
-More wear for parts made of less durable materials/less wear for parts made of more durable materials.
-Damage to couplers or the rest of the locomotive/car if it collides too quickly with another locomotive/car, or if a connected locomotive pulls too quickly/too forcefully.
-Not having sufficient lubrication during operation.
-Damage to wheels from wheelslip, braking too suddenly, or taking on curves that are too tight. (and essentially anything else that creates sparks)

Steam:
-Allowing boiler/firebox to build up excessive amounts of impurities which can also lead to corrosion.
-Using the reverser when in motion.

Internal Combustion:
-If a distinction between different types of IC engines is made, using the wrong type of fuel (e.g. gasoline/petrol in a diesel engine).
-Insufficient coolant for the engine causing overheating.
-Broken turbocharger/supercharger leading to incomplete combustion.

Electrical:
-For locomotives with commutated electric motors or DC traction generators, damage caused by placing too much load on them at low speeds.
-Additional damage to electric motors caused by driving them through water or another conductive liquid.
-Trying to use dynamic brakes (if implemented) if cooling fans are non-operational. This could also apply to other components that require cooling like rectifiers or inverters.
-For electric locomotives, trying to run an electric locomotive on a voltage/current type that it is not designed for (some locomotives are more voltage-tolerant than others). I'm not fully educated on how the physics of this should work, but some good analogies for the results of this would be sticking an LED in an outlet or trying to power a desktop gaming PC using a hand-cranked generator.

Steam: -Allowing boiler/firebox to build up excessive amounts of impurities which can also lead to corrosion. -Using the reverser when in motion.

What would cause impurities in water? Is it RNG based or could players take steps to prevent it? (such as using different water substitutes)

Most natural sources of water contain dissolved minerals, which are deposited in the boiler as the water is evaporated. Eventually this builds up scale which must be cleaned out. It was common to treat locomotive water with additives to reduce the buildup of solids or to make them easier to remove.

Here is a more in-depth description on a smaller scale:
https://www.discoverlivesteam.com/magazineold/190/index.htm

Distilled water, which exists in some MC mods, is 100% water and does not contain these impurities.