Good question! A good train station can be the pride and joy of a city, a bad one can blight an entire neighbourhood.
Of the top of my head, I'd say:
Surrounding area: train stations are the first places many people see of a city, but they can become surrounded by shabby buildings and a focus for drug abuse and street prostitution. I'd say the most successful train stations are the ones that put relatively dense shopping and residential streets very near the stations - cities that do this well are Zurich and Bath, England. Stations that are surrounded purely by offices become dead and intimidating at night, and stations that open up into vast main roads lined with large buildings like Kings Cross in London are a nightmare.
Tracks - you need to make it easy for people to cross the tracks, otherwise, you get a tendency for the area on one side to thrive, and the area on the other to die. For termini (if that's the right word) you need to make a big effort to open out the area behind the station - it's all to easy to make it an industrial wasteland. With the Kings Cross redevelopments in London, a big effort is being made to make it pleasant and easy for people to head into the areas behind the station, where the tracks run in.
Buildings - you've got to build tall interiors. High train sheds and ticket halls make travel exciting, low buildings make you feel like you're in a 1970s nightmare shopping mall. The contrast between St Pancras and Brussels Midi on the Eurostar is a great example of this.
www.enjoy-europe.com/cds/belgium.htm
Enjoy-Europe.com
My pic
Waiting/communal areas - I think the trick here is to get a balance between inside and outside. Overly-enclosed waiting areas tend to smell rancid and make you feel that you are cut off from information about your train. Stations that are too much of a barn inside, however, like Lille in France, are freezing and unhappy. Again, Zurich does a good job striking this balance, with a large comfortable space that hosts a Christmas market and other events, and that has shops and restaurants around the side.
www.zurika.com/2007_06_01_archive.html
Jul, E blogger
Well, that's my 2 cents!