Your problem is not mainly "electronic" but *microphone placement*.
What is the intended use?
Sit at a bar table and eavesdrop a conversation 2 or 3 tables away?
It can be improved, knowing your *exact* use and circumstances, and some electronics filtering can be used afterwards, to improve results, but you need to pick the correct signal first.
Microphones are brutally honest, and if they pick up trains and traffic, it's because that's what is reaching them, period.
So at your table there is 80dB of train and traffic noisebackground and 60/65 dB of interesting conversation?" ... no matter how much gain you apply, the 15/20dB difference will remain and voices will still be buried in noise, only answer is to improve pickup, such as: shotgun/parabolic/very directional microphones, setting mics closer to source, using an RF/ultrasonic/infrared/laser link, etc.
Just for fun, rent and watch this excellent old movie: electronics has improved immensely; microphones not so much (if at all)
https://www.youtube.com/watch?v=vrhRsZ56b4g