Presented in no particular order, these are sites I have found useful over the years either as reference or inspiration for possible projects. (Note US ones added to the US thread).
http://www.canadasouthern.com/caso/ptt/timetables.htm
Despite the name, lots of reference material on the New York Central including passenger and employee timetable. (Oops, should have been in US!).
http://www.sporenplan.nl/
Diagramatic track plans for a large proportion of the European railway system, including some historical data.
http://www.sa-trackandsignal.net/
Australian infrastructure site.
http://www.nswrail.net/
Detailed synopsis (including track plans) of most New South Wales railways including historical.
http://www.victorianrailways.net/index.html
Lots of information on Victoria (Australia) railways past and present including track charts and gradient profiles.
http://www.internationalsteam.co.uk/internat.htm
General info site but includes a comprehensive survey of Russian narrow gauge lines.
http://www.michaelclemensrailways.co.uk/
Sundry GWR info including timetables and plans.
http://www.ribblevalleyrail.co.uk/Old%2 ... les.htm#SW
Various historical timetables for UK & Ireland.
All the above worked at the time of posting, sadly the Penmorfa site with lots of info on the old Yugoslav NG seems to have gone.