The picture above represents only part of the circuit. Click it to see it in full and in high definition. Attention: the etching mask must be turned over for etching the tracks.
I must confess to you something else: I use indirect lighting. That is, the LEDs are directed towards the ceiling, and the light is reflected by a reflector stuck to it. Avoid household aluminium, conductive. I found a very convenient and inexpensive product: the space blanket. Attention: it also has a conductive side, but not always the same: you will have to test.
Advantages of this method:
The only disadvantage I see is the room sometimes narrow under the ceiling of some cars. It is necessary to keep a certain distance between the top of the LEDs and the roof (at least 5 mm).
Here are some aspects of this circuit.
In this picture we see the end of the circuit (this is the driver car), with the decoder green and blue wires, as well as the resistor that powers the lamps. The decoder is located under the driver. There is, in fact, a large space in this area, only occupied by outgrowths which were used to maintain the light circuit bulb, in the prehistoric times when incandescent lamps were still used. After having levelled all this, we obtain a perfect housing for the decoder, masked by a plastic plate on which the conductor is glued. Note a semblance of decoration drawn on a cardboard, glued on the wall of the cab. Must be improved, no doubt!
In the following picture, we see the capacitor circuit, mounted in the air, because I had not thought it necessary at first. Notice the transparent plastic partition that I had to install to support the PCB. I put three in total. Note that since then, I have designed a small independent “energy tank” circuit, with or without a reed switch, which can be adapted to almost any car.