Web scraping XML with beautifulsoup

Question

I've been trying to understand the principals of web scraping, with some difficulty. I am trying to write a script using beautifulsoup that scrapes the data for train schedules (found here http://api.bart.gov/api/etd.aspx?cmd=etd&orig=rock&key=MW9S-E7SL-26DU-VV8V) and tells the user when the next train is arriving. I've already gotten the data in with urllib2 and beautiful soup: I can print it out and see it, but I'm having trouble writing an algorithm for extracting that information. Advice greatly appreciated.