I thought I would start a thread for Houston's light rail system. The current Red Line is about 7.5 miles long and is one of the busiest light rail lines in the US. It is currently undergoing an expansion with the Red Line extension now scheduled for completion in the fall of 2013 along with the Green Line in 2014.
Map of the proposed/under construction light rail expansion.
Map of the proposed/under construction light rail expansion.