The problem with sound waves (and it is what i deal with in my day job) is that the multiple and differing sources of them make cancellation techniques using reproduction of a phase shifted variant of the original sound pretty much impossible to achieve. Absorption is the only feasible technology we have for this when we are talking about the full frequency spectrum. Structure bound noise like the low frequencies found in these (and all) engines are where the problem becomes complex.
You can block out lots of noise using physical barriers but these as has been described already come with their own problems. but the lower frequencies (what you feel instead of hear) are more difficult to minimise. Lower frequencies do travel EASIER through the air / objects so they are harder to block out, despite this they reflect off surfaces too meaning if you are sat on your garden a few feet from your house then lower frequencies will appear louder. Turn your hifi on and listen to your favourite music in the middle of the room, now stand against the rear wall, now in the corner, each time the low frequencies appear louder. The only real way to combat this is to treat surfaces acoustically and an extra pane of glass doesn't help this (although it will help the higher frequencies and noise in general so don't give up on that quest).
As humans we often cannot tell a difference in a couple of dB especially with complex wave forms (not a single tone) so if a train is 1dB louder we likely wouldn't tell. Where we WOULD notice is the content of that noise for example, an engine could be quieter but contain more lower frequency noise and this would cause more "off site" issues through structure bound noise.
Remember also that 2 trains together doing exactly the same thing will make it louder, but not in a linear fashion. in some areas part of the noise would be quieter due to the way sound waves work, but in other parts it would be louder, so there is no one size fits all solution here either.
To drift slightly off topic but to give a relative example. In my job we frequently have to work to environmental noise limits which can often be barely achievable. Quite often on a festival site there are 1 or 2 local residents that do not like the noise from the music at a festival and it is our job to try to minimise this whilst still ensuring people who are attending the festival can hear at a respectable level. There are lots of ways of doing this, all of which are time consuming, complicated and expensive as it usually involves more equipment and more hours doing calculations. The local residents have a right to the same peace and quiet they get the rest of the year so everyone tries to work together to achieve the best results for everyone.
A more "all year round" example that relates further is that of the couple that move into a house next door to a live music venue and complain of the music they can hear and feel in their property. They do have a right to complain, and the venue should assess the situation and take any steps they can to minimise this. Sadly our legal system seems to work in such a way that many venues are forced to close because of this costing hundreds of jobs. I'd like to think the railway has a bit more clout than this but on the other hand it also has a bit more budget to try and find a solution.
