Let's start by converting the velocities of both trains to m/s:

Train A: $$108 \frac{km}{h} \times \frac{1000 m}{km} \times \frac{1 h}{3600 s} = 30 \frac{m}{s}$$

Train B: $$72 \frac{km}{h} \times \frac{1000 m}{km} \times \frac{1 h}{3600 s} = 20 \frac{m}{s}$$

To cross the tunnel, Train A has to cover a distance equal to the length of the tunnel plus its own length: $$L + l$$

Similarly, Train B has to cover a distance equal to the length of the tunnel plus its own length: $$L + 4l$$

We are given that Train A takes 35 seconds less time than Train B to cross the tunnel. Let's denote the time taken by Train A as $$t_A$$ and the time taken by Train B as $$t_B$$. Then, we have:

$$t_B = t_A + 35$$

Using the formula distance = velocity × time, we can write the equations for both trains:

Train A: $$(L + l) = 30t_A$$

Train B: $$(L + 4l) = 20t_B$$

Now, we can substitute $$t_B = t_A + 35$$ in the equation for Train B:

$$(L + 4l) = 20(t_A + 35)$$

We have two equations and two unknowns ($$L$$ and $$t_A$$). We can solve this system of equations by eliminating one of the unknowns. Let's eliminate $$t_A$$ by expressing it from the equation for Train A:

$$t_A = \frac{L + l}{30}$$

Now, substitute this expression for $$t_A$$ in the equation for Train B:

$$(L + 4l) = 20\left(\frac{L + l}{30} + 35\right)$$

Multiplying both sides by 30 to get rid of the fraction:

$$30(L + 4l) = 20(L + l) + 20 \times 35 \times 30$$

Expanding the equation:

$$30L + 120l = 20L + 20l + 21,000$$

Simplify:

$$10L + 100l = 21,000$$

Since we are given that $$L = 60l$$, substitute this into the equation:

$$10(60l) + 100l = 21,000$$

Solve for $$l$$:

$$700l = 21,000$$

$$l = 30$$

Now, substitute the value of $$l$$ back into the equation for $$L$$:

$$L = 60l = 60 \times 30 = 1800$$

So, the length of the tunnel is:

1800 m