How much sound colliding objects make depends entirely on the objects and the medium they are in. In the vacuum of space collisions don't make a sound, at all. In Earth's atmosphere the total energy of sound released by collisions that are caused by solid objects is very small compared to the energy of the objects. This is because of the large difference in density between solids and the atmosphere.
In water, however, this is entirely different, again, because the density of water is of the same order of magnitude as the density of the solids. The "sound" of an underwater earthquake is a tsunami and we have seen twice recently just how much energy can be set free in form of those tidal waves.
In short, there is no general formula, it all depends on the details of the collisions and the answer can range from nothing to near 100% of the incident energy if the densities of objects and media are matched.
Every object moving through air has a wave front, like a boat moving through water. The faster the object is moving, the larger the wavefront and its amplitude.
When two wavefronts run into each other, they add their amplitudes. The wavefronts are slightly behind and to the sides of the fronts of the trains. When two trains pass by each other at high speed, the wavefronts collide between the trains, adding together.
Also, as the fronts of the trains pass each other, they are compressing the air between them, which also adds to the amplitude of the wave (it's a compression wave after all), something that cannot occur without another train, or possibly a very close tunnel wall. The sides of the trains, as they are moving quickly, invoke the Bernoulli principle, resulting in low pressure between the trains to contrast with the high pressure around the fronts of the trains.
If the trains are moving fast enough and are the right shape, then a sound similar to a slapping sound will be produced between the trains as they pass. If they are each moving at about half the speed of sound, then the resulting slapping sound would actually be similar to a sonic boom.
This occurs only between the trains, because it is only between the trains that these circumstances occur. On a platform, you won't hear it because the train is blocking the sound. If you could stand between the closely passing trains (somehow), you would hear the sound. If you're on a platform between the trains though, it won't work because the trains are too far apart; their wavefronts don't reach very far.
Here is a link to a slide show that simplifies a long paper written on this very topic. If you would like the whole paper, you can find it here.
Below is an image depicting the solutions to the pressure equations. It is simple to see how the high pressure in front of the red train would push on the windows and body of the blue train. As the front of the res train passes, the quick transition from medium to high to low pressure would likely snap the windows and outer body of the blue train enough to produce the aforementioned "slapping" sound on the inside of the car. This sound is accentuated by the spaces between cars, and especially the front of the train.
To be clear, this paper is probably the best explanation you're going to get on this topic. I've done my best to understand where this described "slapping sound" would come from based on that. The only way I can think of getting more information is to do a more complex simulation or a slow motion video of the outside of the train as the other passes by. It would be cool to see the ripple across the body of the train as the other train passes.
EDIT: To clarify, there are two sources of sound here: 1 - The fronts of the trains hitting each other. And 2 - The pressure difference creating a ripple on the outside of the body of the trains.
Best Answer
Sound is longitudinal pressure waves in the air, It can be produced in a couple of ways in a collision.
The impact can set up pressure waves and or ringing in the bodies themselves which then interact with the surrounding medium to produce audible sounds. This is the mechanism of a bell.
If enough air is forced out of the space between the two bodies as they approach sound can be made either directly (the displacement is the initiating event) or a secondary effect of that out-rushing air interacting with the edges of the bodies. This mechanism is rare enough in everyday life that I can't think of an example off the top of my head.