Sound insulation materials work by using their impedance to reflect sound waves, resulting in very little transmission of sound in the area covered by the insulation material. On the other hand, sound absorption materials achieve an infinite sound field through their absorption structures and media, thereby reducing reflected sound waves. The application of these two types of materials has different requirements. Simply substituting one for the other may not meet your technical specifications and could even have the opposite effect.
A more practical example would require the application of the theory of sound field modeling for analysis, using some related equations to solve it. Take what you mentioned as an example. If sound-absorbing materials are used in a concert hall, the hall is originally designed to balance the reflected sound field and the infinite far-field. Appropriate sound-absorbing materials are used to eliminate unnecessary reflections and achieve a purposeful reverberation field. However, if sound-absorbing materials are replaced with soundproof materials, the sound that was intended to be weakened is reflected back, causing a change in the reverberation field. This might result in the music you hear being a loud buzzing sound, and it persists constantly. Generally, in a concert hall, sound-absorbing materials are selected based on the architectural structure, main function, and desired effect of the hall, and they are designed to absorb and weaken sounds at different frequencies. These are the main purposes of architectural acoustics.
The situation regarding the use of sound-absorbing materials along the railway is as follows. First of all, let me clarify that sound-absorbing materials do not mean completely eliminating the sound; instead, they consume the energy of sound waves at certain frequencies. However, sound waves at other non-absorbing frequencies can still pass through the material. Railway noise has a wide range of frequencies and a large amount of energy from the source. If only using general sound-absorbing materials, the effect would be negligible. Behind the set sound-absorbing materials (usually in residential areas), there is still a lot of noise. And sound insulation materials are generally reflective materials; they can almost completely reflect the incident sound waves back.
Of course, in terms of special design, the noise reduction on the railway side can also be achieved by using sound-absorbing materials. Human hearing is sensitive to noise at certain frequency ranges. By taking advantage of this, it is possible to set up the absorption of sound waves at these frequency ranges to achieve the effect of eliminating noise. The most commonly used methods are probably pure reflection and a combination of the two. I'm not involved in noise reduction for railways, so I don't know exactly what they use, but the basic principles of sound absorption and reflection treatment are basically these. The remaining part is about how the material design specifically eliminates the energy of sound waves.