This mod attempts to make the steam locomotives more realistic, by changing three things:

• the throttle calculations,
• the superheater on the S282,
• and the way water enters the boiler on the S282.
  

I've done a lot of research for this but I've never run steam locomotives in real life. If you have real-life experience with steam locomotives, and you think I've made a mistake, please let me know.

Throttle calculations

In vanilla Derail Valley, the throttle on the steam locomotive regulates the pressure in the cylinders. But in real life, the throttle limits the rate of steam flow into the cylinders. This means that even a small throttle setting can take the cylinders to the full boiler pressure; it'll just take slightly longer to get to full pressure. If you charge the cylinders to 14 bar, then close the throttle, the cylinders will stay at 14 bar until you use that steam. Engineers will regulate the pressure in the cylinders by opening and closing the throttle quickly, before the cylinders can charge to full pressure.

This mod changes the behavior of the throttle, so that it correctly limits the rate of steam flow into the cylinders. This means that at low speeds, especially in the rain, you might get wheelslip even at the lowest throttle setting. To combat this, quickly open and close the throttle (either by using scroll wheel while highlighting the throttle, or by using the T and G keys while in the locomotive). The steam chest pressure gauge helps a lot: if it shows that you're getting wheelslip at 9 bar, you can try only charging the cylinders to 7 bar.

Note that it's now very hard to start the S282 when on a hill and in the rain, even while hauling nothing - as it should be.

Note that the locomotives have different steam chest sizes: the S060 has a dome throttle in the steam dome, and the S282 has a front end throttle in the smokebox (throttle linkage is inside the boiler). So the S060 actually has a larger steam chest than the S282, even though the S282 is superheated (the superheater is before the throttle, so it doesn't effect throttle response).

TLDR: the throttles on both the S060 and the S282 are gonna feel weird and annoying. Watch the cylinder pressure gauge; you might need to open the throttle in short bursts when starting to avoid wheelslip.

If the throttle feels weird in VR, let me know; I don't have a headset so I can't test it.

Feedwater heater

The S060 and S282 simulate different methods of adding water into the boiler: the S060 uses an injector, and the S282 uses a feedwater heater. (It doesn't look like it is from looking at the S282's model... but then again, there's no injector modeled either). An injector uses steam from the boiler to push water from the tender into the boiler (using the venturi effect, aka black magic). A feedwater heater uses exhaust steam to heat up the water, then a pump powered off of boiler steam to pump water into the boiler. Both systems heat up the incoming water, but feedwater heater systems are more efficient because you are using exhaust steam to heat up the water, instead of using boiler steam to heat up the water.

For the most part, this is correctly simulated in the game: the S060's injector is much less efficient than the S282's feedwater heater. However, the efficiency of a feedwater heater depends on how much steam is in the exhaust: if the locomotive isn't using any steam, then the feedwater heater can't run off of exhaust steam and has to run off of boiler steam, negating the efficiency benefits. So the feedwater heater should only be efficient if the cylinders are actually using steam.

This mod fixes this issue: on the S282, the feedwater heater is now only efficient when the cylinders are exhausting steam, and very inefficient when there's no steam in the exhaust.

To be honest, I don't really like the idea that the S282 has a feedwater heater, even though that's what devs have told me... I might try and give the S282 an exhaust steam injector at some point. I would also like to properly model the injectors too... but that's going to have to wait.

TLDR: try not to put more water in the S282's boiler unless there's steam in the exhaust. The S060's injector is unchanged.

S060 volumetric efficiency upgrade

In vanilla Derail Valley, the S060 becomes very inefficient above about 60 kph. This is understandable; the developers probably wanted to limit the speed of the S060 without making it unstable at high speeds. But it's not super realistic: the S060 has the same size drivers as the S282, so it should have roughly the same top speed.

To fix this, I've given the S060 the same volumetric efficiency curve as the S282, but a more restrictive throttle. This means that the S060 will be as fast as a S282, but it's far less powerful at those high speeds. Along with the S060's smaller cylinders, in my opinion this results in a balanced but still vaguely realistic S060. It's a little trickier to drive though: it's a little too fast for its own good. Considering that not many 0-6-0's had wheels as big as the S060's wheels (56 inches), this seems suitable.

This also makes the S060 a good passenger engine, if you're using the Passenger Jobs mod.

TLDR: S060 fast, because the drivers are the same size as the S282's drivers. The tractive effort and tonnage of the S060 have stayed the same.

Adjustable max boiler pressure

You can now adjust the max boiler pressure of the S060 and S282 in the settings. Keep it below 30 bar; the steam simulation is only accurate to 30 bar and wacky glitches can happen above that (all boilers instantly explode at about 60 bar lol)

S282 superheater

The S282 has a superheater in vanilla DV, but its simulation is a bit simplistic; the game just uses the firebox temperature as the temperature of the steam in the cylinders. This means that in vanilla:

• When the fire is cold, the superheater can sometimes cool the steam (which I think is impossible in real life)
• When the fire is hot, the engine is far too efficient (real superheaters only get up to about 400° C; the vanilla S282 superheater can get to 1400° C!).
  

This mods adds a more detailed superheater simulation. The amount of superheat now depends on:

• The temperature of the fire (hotter fire = more superheat)
• The airflow through the flues (more draft = more superheat)
  

I've also made it less efficient than the vanilla superheater to match what real superheaters can achieve. If you want the OP superheater from vanilla, you can change the superheater effectiveness in the mod settings (0.3 is vaguely realistic, 1.0 is vanilla).

Some nerdy details that don't really matter:
TLDR: S282 is more efficient when the fire's hotter, and when there's more draft. The superheater is also less OP; if you don't like that, you can change it in the settings.