This will be on the test so...
I hooked 12V DC to a LM2596 modules w/voltmeter to the worklight LED array of 24 bypassing the resistor to directly drive 24 parallel white LEDs (5mm diameter).
You will probably get similar results and will probably find you can operate closer to 3.0V. If you're a student, here's that sharp diode-junction "knee" or hockey-stick on the V-I curve in action which is the first curve they always show in the chapter on semiconductors. That is, note the small changes in voltage having an outsized effect on current going from 2.8 to 3.0 to 3.2 Volts. For example, going from 2.8V to 3.0V (delta of 0.2V) causes the incremental current per LED to increase about 7 mA. Use Ohm's law and you get the dynamic resistance of the LED at that operating point...much smaller than the average resistance of the LED. Hmm, I hope they teach you about dynamic resistance and load-line curves!
The big win with switch-mode regulators is conversion efficiency. So at 3V, the 24 LEDs pull 0.3 Amps or ~0.9 Watts. At that output voltage, the 12V input current was 85 mA or ~1.0 Watt. So the efficiency (power out/in) = 90% which is very good for this type of power converter. Actually a little power is being consumed by the voltmeter circuit but can be ignored for this example. If you used the resistor method, the current in and out is the same. So for the same 0.9 Watts out, the input power would be 12V * 0.3 Amps = 3.6 Watts. The conversion efficiency would be 25% which is not so good as 75% of the energy is wasted as heat.
I have no idea why there's a USB connector on your module. I found an eBay listing that looks like your module and one photo shows the USB connected with the voltmeter at 5.0V. So perhaps this is meant to be a USB charger...though it seems pretty risky to require a user to set it to 5.0V before using it as a charger?! Anyway that's my guess.
WRT to DCS signaling, the inductor isolates the impedance (not resistance) of the capacitor from the track voltage. The DCS signal rides on (in class they'll say "super-imposed on") the low-frequency 60 Hz track voltage and is hi-frequency in the kHz-MHz range. A capacitor's impedance is inversely proportional to frequency so it tends to load down the DCS signal. An inductor's impedance is proportional to frequency so by placing it between the track and the circuit, it demotes the loading while still allowing the power signal to reach the load.
Separately, your tunnel photo shows lights on each side of the track. Rather than a single long string of 32 LEDs, run two strings of 16 each (or whatever) from the converter output. This cuts the max current running down each string in half which is beneficial for voltage drop effects as well as wire heating though with the current levels involved you probably won't see any effect.
