One of the first comments I received when I announced I was working on a model of the Clayton was that the light mouldings really were crying out to be fitted with working lights, and who am I to refuse such a sensible suggestion. The problem is that while the mouldings are big in relation to the size of the model they still aren't very large. Fortunately LEDs are available that will fit but they are very very small.
The smallest surface mount components I've used before are the resistors in the Hudson-Hunslet, which come in a 1206 package. They are known as 1206 as that is the imperial measurements of the component, i.e. 0.126" by 0.063" or in metric just 3.2mm by 1.6mm. For the lights on this loco I've had to go for LEDs in a 0402 package which is 0.039" by 0.020" or an eye wateringly small 1.0mm by 0.5mm. Given that those dimensions are for the entire LED you can probably imagine just how small the two contacts are that you need to solder wires to. My eyesight for close work might be quite good and I have a brilliant magnifying lamp on my desk but I don't think I would have the patience or dexterity to wire these up. Fortunately you can buy them pre-wired on eBay! This helps but the wire itself (referred to as magnet wire) is also ridiculously thin so it's still a fun challenge wiring these up.
Of course there isn't just the LEDs to wire in, but a resistor to protect them from the full track voltage (2.2K Ohm in this case) and a capacitor to help reduce flickering all of which need to be connected together and then stuffed into the small space behind the motor.
As you can see there is quite a bit of stuff, even if some of those wires were trimmed back before I finished) to get into the body so there was a fair amount of careful stuffing involved. Once the body was on the result though is this.
I could probably have used a slightly bigger resistor to drop the brightness a little further but for a loco that originally worked in a long dark tunnel this seems okay, and a bigger value capacitor might have reduced the flickering a little further, but in general I'm really happy with the result.