Solution:
Let the length of the tunnel be 'x' meters.
Let the speed of the train be 'v' km/hr.
Let the time taken by the train to cross the tunnel be 't' minutes.

Conversion of units:
As the speed of the train is given in km/hr and the time taken to cross the tunnel is given in minutes, we need to convert them into a common unit. We can convert the speed from km/hr to m/min and the time from minutes to hours.

- Speed of train = 36 km/hr = (36 x 1000)/60 m/min = 600 m/min
- Time taken to cross the tunnel = 9/2 minutes = (9/2)/60 hours = 0.075 hours

Calculation:
We know that distance = speed x time (when the time is in hours and speed is in meters per minute).

- Distance covered by the train in crossing the tunnel = Length of the tunnel + Length of the train

Here, the length of the train is not given. But we can assume it to be 'l' meters.

Hence, the distance covered by the train in crossing the tunnel = x + l meters.

So, we have the equation:
x + l = 600 x 0.075
x + l = 45

Now, we need one more equation to solve for two variables (x and l).
We know that the time taken by the train to cross the tunnel is the same as the time taken by the train to cover the length of the tunnel and the length of the train.

- Time taken by the train to cover distance (x + l) = 9/2 minutes

Here, we can use the formula of time, distance and speed to get another equation.

- Time = Distance / Speed

So, we have the equation:
9/2 = (x + l) / 600

Solving the Equations:
We have two equations and two variables. We can solve them by substitution or elimination method.

If we substitute l = 45 - x in the second equation, we get:
9/2 = (x + (45 - x)) / 600
9/2 = 45/600
9/2 = 0.075
Which is not possible.

So, we need to use the elimination method. We can multiply the second equation by 600 and simplify it to get:
300x + 300l = 2700
Simplifying the first equation, we get:
l = 45 - x

Substituting this value of l in the above equation, we get:
300x + 300(45 - x) = 2700
300x + 13500 - 300x = 2700
13500 = 2700
Which is not possible.

Conclusion:
We cannot solve the given problem as the equations obtained are inconsistent. Therefore, there is no unique solution to the problem.