I was impressed by a recent piece of research by the Center for Transportation Studies at the University of Minnesota. It looked at the impact of the Hiawatha Light Rail line, running south from Minneapolis to Minneapolis-St. Paul airport and the Mall of the Americas.

"The Hiawatha Line: Impacts on Land Use and Residential Housing Value" by Edward G. Goetz, Kate Ko, Aaron Hagar, Hoang Ton, and Jeff Matson, was published in February 2010 as Report no. CTS 10-04.
It can be found on the website of the Center for Transportation Studies at the University of Michigan

Three major questions are investigated -
what were the impacts on property values of proximity to a Hiawatha line station,
how have land uses changed around stations, and
what are the impacts of the stations on the level of housing investment within the corridor?

All of these were answered positively.

The line runs through diverse neighbourhoods.

Homes within half a mile of a station increased in value more than a control set of homes - although the impact is greater where the stations are more accessible. On the east side of the line, the 4-lane Hiawatha Avenue, fringed by a strip of industrial property, pushes residential areas away from the line.

Significant new housing construction has occurred in the catchment area of the line.

It's a good study showing that there are trivial negative effects on houses very close to the line and significant positive ones on others nearby.