Here's a link to MEXILIST, a website that has lots of information about the Mexican railway system...
Here's a direct link to Mexilist's directory of railway lines in Mexico
Here's a direct link to the photo gallery
Canadian Pacific has a small blurb about the Mexican railways on their site as well -
http://www8.cpr.ca/cms/English/Customer ... lroads.htm
Hope that's enough to get you started.