After looking at a map I see the hotel is next to train lines... Is there much noise from the trains? Am a rather light sleeper!
I slept in a room near the rail lines and heard nothing at all, awake and asleep. The trains are approaching the station and so are moving at snails pace past the hotel.
Having stayed in this hotel recently I found it a good standard in every way. My room was overlooking the rail lines. Not once was I disturbed with any noise from the rail lines or traffic. Myself and family had a lovely stay.
It wasn't that noise, I'm a light sleeper too. I guess it depends on whether you get the room which is facing the rail. Otherwise it's ok, and the location is very convenient
Get an answer from Mercure Paris Saint Lazare Monceau staff and past guests.