This is real life issue I am having currently and I am puzzled as to why it happened. I posted the question in the engineering section but it was put on hold as too broad. I have no other way to narrow it down as it is a real life ongoing situation. I have lived over the subway lines (two actually each going in the opposite direction, two sets of tracks) for two years. The noise has always been a soft background hum, with an occasional louder train (that usually needs wheel maintenance), hardly noticeable. One early morning near the end of January I was jolted out of bed by a loud noise. Turns out it is the subway. It is as if someone turned the volume up from 3 to 8 overnight. Noise much louder (both tracks), last longer and there is shaking in the house too. Some small variances in the loudness of each train, but all much louder. The quietest train now is much loudest then the loudest one was before. I know that no changes were made to the tracks, trains, schedules or my building. What else could have caused it - noise from subway became much louder overnight? What could have changed the way sound travels between the tracks and my house?
Assuming your observation is correct, and does not depend on the way you hear, (sudden clearing of ears for example), the only solution is change in the condition of the ground which insulates the sounds coming from the trains.
The only overnight change I can think of is water damage, a broken pipe changing dry ground to mud. There exist studies of sound propagation through different humidity soils, but too hard for me to understand cursorily. It seems to me mud is more insulating.
Was there a very large storm preceding your observation? Leaking storm drains , or backup from river could have the same effect.
Water leaks could also suddenly cause cavitations, which would increase air pockets and bring the sound closer to you. I would ask the train people to check for water leaks into the tunnel .