Adding a firebox glow/flicker light to a steam loco produces a great effect.
You'll need an orange LED of some sort, resistor, and a suitable decoder that can support a glow or flicker lighting effect. All but
the most basic decoders these days provide such a feature.
Some people prefer a random flicker lighting effect, but in reality the light coming from the firebox is more of a steady glow, so
a subtly glowing lighting effect strobing between say 100% and 60% is very pleasing.
Depending upon your skill level, you can either go for a cheap, simple domed orange LED, which is very easy to fit, or go for a flat SMD
(surface mount) LED, which you can get in a better orange colour, and produces a far more intense light. The cheaper LEDs look pretty good anyway,
but tend to be a bit more reddish in colour.
Either way, drill a hole of around 2mm directly through the firebox, and glue the LED behind. The domed LED is much easier to glue because
its dome will slot into the hole you drilled. To improve the light, you may want to file out a slight recess from the inside of the loco's body,
which is particularly useful if using the SMD LEDs, as they can then sit closer to the visible apperture, and the light is more obvious.
Beware that some models have lots of metal around the back of the firebox, so check that your model is suitable for this feature, and be certain
that you insulate any electrical connections on the LED.
You must ensure that a resistor (e.g. 1k) is attached between the positive terminal of the LED and the BLUE decoder wire. The negative terminal of
the LED can then be connected to one of the decoder's function output wires. GREEN or PURPLE are the most commonly used ones for this purpose,
although any output will do, as long as you can configure the lighting effect, and make it work in either direction of travel!
As an example, if you use a CT decoder, and want to use the GREEN wire output, you then need to set CV156 to a value of 9, which means
'Gyralite' lighting effect. There are other CVs that affect the low and high points of the glow effect, as well as the speed of the pulses.
Note that other decoder manufacturers will use different CVs, so check your decoder's manual!