Are the purlins in a pair of semis all in one piece, i.e. continuous
through
the party wall? I ask because I think my neighbour has a train set in his
loft, and the slight noise / vibration can be heard in mine. It is not a
problem, just curious!

The purlins in our loft (1900's) overlap between adjoining lofts:-

http://www.pciq.co.uk/pics/purlinwall.jpg

As Set Square says, doubt it would be a single span across two properties
though.

Andy