Sorry about your name spelling. I don't know how to type accents on my keyboard.
I have a new in box Titan FX-DO and just had a quick look. There is a error on the drawing for the bottom board. Perhaps Kelly could have a look at this. Connector P2 is mislabeled "TRUCKS"; when, in fact, only pins 1 and 2 are for the trucks (as are the schematically parallel P1 pins 1 and 2). Connector P2, pins 3 and 4 are to be connected to the motor (as are the schematically parallel P1, pins 3 and 4).
That being said, the wires from P1 are labeled correctly.
If you look at the bottom of the bottom board, you will notice a large black box component with a hole in the middle. That is a heavy duty, full wave bridge rectifier. If you understand how a bridge rectifier works you will know that the voltage across the + and - side (output) is track voltage minus 1.4 volts. In your case you will have 24 V - 1.4 V = 22.6 V total.
Kelly recommends using an opto-isolator between the lighting and the top board. This is the safest route as the expensive decoder board is protected. The input side of an opto- isolator is internally similar to an LED. You connect one side to plus +5 V and the other is connected to the decoder light port via a current limiting resistor. The output side of the opto-isolator can then be used as a simple switch. N.B. the isolated circuit, controlled by the output of the opto-isolator, must have the GND connected to the GND of the controlling circuit (the "-" side of the full wave bridge rectifier - same as the top board P1 - 8 and P2 - 8).
A note about LEDs in series. While red LEDs run around 2 volts, white LED takes about 3 volts to turn on (some actually take 3.1 or 3.2 V to turn on). Current controls the brightness. That's why it's absolutely mandatory that a current limiting resistor be included in any LED circuit. A +5 volt circuit can only drive one (3 Volt) LED. However, a +22.6 Volt circuit can drive up to seven LEDs; 3 V X 7 = total 21 Volts (which is less than 22.6 Volts leaving room for the current limiting resistor).
We must choose our maximum current for the LEDs. Most electronics engineers recommend a current maximum of 20 mA. I use varied current depending on the LED function. Headlights are brighter than ground lights. For Headlights I might use 15 mA (very bright) while a ground light might only require 2 mA (considerably dimmer).
Let's do some math. Please excuse the simplicity, but this is a forum and our audience may not know how to calculate resistance:
e.g. 22.6 V - (7 X 3 V) = 1.6 V We now use Ohm's law to figure out the resistor value for a 2mA circuit. Volts = Amps X Ohms or Ohms = Volts / Amps
1.6 V = 0.002 Amp X Ohms or Ohms = 1.6 V / 0.002 We get 800 Ohms. If we don't have the exact value resistor we choose a value slightly larger. We can use 1000 Ohms (1K Ohm, 1/8 Watt) with no problems.
If you decide to add more LEDs, you may add a second series circuit in parallel with the above series circuit. Use the same formulas adjusting for quantity of LEDs in the series circuits.
I hope this helps. Let us know if you have any questions (or corrections).