I've seen this question pop up a few times, "why do I get sparks out the exhaust of my steam engine?" And I wanted to give a good answer!

You'll notice these sparks when you have the regulator near or fully open. This is sending the maximum amount of steam to the cylinders to provide acceleration. A lot of people ask if the sparks cause damage or what they do/cause and why they get them at all.

To answer that, you need to know a bit about how a steam locomotive produces steam. Obviously, there's a fire which generates heat for a boiler which boils water into steam. But, critically, as an engineer, you control the fire. You can control how much air it's getting and how much fuel it has. The more of each, the hoter it becomes.

In both locomotives, you'll notice something called the damper. But, if you're like me, the first couple of times you have played with it, it doesn't seem to do much. What the damper does is allows air passing by the train to be sucked into the firebox. The more up the lever is, the more open the damper is. The firebox pulls air in through the damper, while at speed, thanks to a draft. A draft is generated by either the blower (which doesn't really work past 5kmph) or by the blast valve.

This valve redirects pressure from the boiler to generate draft inside the firebox, thus drawing in air through the damper to raise the temperature. Critically, this ONLY works when the regulator is open! So, if you are coasting, you'll notice your temperature drop despite the damper being open. That's because this valve isn't producing any draft in the firebox. You open the regulator back up, you'll notice your temperature increase.

Next, you'll also need to understand the color of smoke your locomotive is producing. If it's black, your running very rich, there's not enough air going into the firebox. This is fine if you are coasting. If the smoke is white, you are running very lean, you have more air than fuel. An ideal color is gray to white, this is where you are utilizing all available heat from your coal and have sufficient airflow.

Finally! Onto what causes sparks! You'll typically notice that you can get pure white smoke along with the sparks! This is because the sparks are actually still burning bits of coal. There is so much draft that you're actually blowing coal out of the firebox (the sparks.) And you notice this the most when the regulator is full open (or nearly) and that valve is creating a significant draft in the firebox, especially at high speeds.

These sparks don't hurt the engine, but they are wasteful and lead to a more costly coal bill when servicing. What you need to do is feather the damper. Push it in enough to stop the sparks and get a nice solid gray smoke. Push it too far and you will not have enough air and will cause your temperature to drop and slowly starve the boiler.

Hopefully this helps and/or gets more people interested in driving these amazing locomotives! They are incredibly rewarding and fun to drive.

If you have questions, let me know! Thanks