I think the main reason I could sleep through the sirens was the good quality of the double glazing; it was one of the selling points of the flat when I was looking, that the landlord was in the process of putting double glazing in. However, I mostly thought of it in terms of heat economy and thermal comfort, but having had the all the windows open on some nights after incredibly hot summer days to try to cool the flat down enough to sleep in, I realized they were very effective at sound isolation as well.

I am not quite sure why some of 'my' trains whistle; it is generally the through-trains, but not all of them do it. We haven't got any level crossings near here, so I don't think it is people trying to cross when the train is approaching; it could be people standing too close to the platform edge, though.