Seattle's Sound Transit filed suit yesterday to condemn a small strip of vacant land in Renton needed for track and signal upgrades, allowing for expansion later this year of the agency's Sounder commuter rail service. The land, appraised at $30,000, was priced at as much as $100 million by a city of Renton official. The Renton City Council is withholding an easement for track and signal improvements that would allow for a third Sounder run between Tacoma and Seattle. The city's staff earlier stated that the land was not needed by the city and recommended the city council approve the easement for $30,000. Sound Transit has evaluated other options for completing the necessary track work. The least expensive alternative to condemning the property in question would cost taxpayers within the transit district an additional $800,000.