I found this schematic,
which in theory alternates the LEDs like in railway crossing lights. First of all I was wondering if it works, and if it does, what frequency does it have, and what would I have to do to modify the said frequency?
The output (pin 3) is alternately charging and discharging the cap. If the output is high, the cap charges up. When it reaches about 6 volts, the internal flip-flop is reset, pin 3 goes to zero, and the cap starts to discharge. When the cap reaches about 3 volts, the flip-flop is set, and the cycle repeats.
As the cap voltage rises and falls, the two LEDs are alternately brightened and dimmed but in opposite directions.
Ignoring the base currents for the transistors, the time to charge or discharge will be on the order of 2/3 RC, with R in ohms and C in farads. Change this by changing C. Do not, under any circumstances, increase R. If you do, you can get into problem with the transistor base currents causing your system to freeze. If you find that this does happen anyways (and it might - I haven't done the detailed analysis to find how likely this is) try reducing the 20k.
A simulation in LTSpice suggests that the circuit will "freeze" (will not alternate) with the prescribed 20k resistor and 220uF capacitor. I recommend switching to a 10k resistor and a 440uF capacitor instead.
As for the rest of the circuit, I suggest bumping the LED current limiting resistors to around 480 ohms. 150 ohms will allow upwards of 48mA through the LEDs (depending on their forward voltage drop) which will most likely destroy them. Keep it closer to 10-15mA.
The voltage at pins 6 & 2 when running as an astable multivibrator is a triangular-ish wave from 1/3 Vdd to 2/3 Vdd, so in this case from 3V to 6V with a 9V supply. Graphic from this website:
When connected with a single resistor from pin 3 (assuming a CMOS version of the 555) the duty cycle will be close to 50% so t1=t2= T/2.
The LEDs are fed via emitter followers so the voltage at the LED will vary from about 2.3V to 5.4V, which, with a 9V supply, would mean the LEDs do not turn off completely. This can be adjusted by reducing or increasing the supply voltage or by adding a series diode for each LED.
The plus side of doing it this way vs. connecting the bases to pin 3 is that the slow variation in brightness more closely matches the visual appearance of a railway level crossing which traditionally have very slow incandescent lamps (run at low temperature because long life is required).
This circuit is a bit marginal, I suspect it would work better with a lower supply voltage such as 5V.