WASHINGTON, Aug 30 (Reuters) - U.S. groups representing transit systems and city leaders on Monday joined unions and environmental groups in calling on Congress to back at least $10 billion in additional public transit spending along with new funding for high-speed rail. The American Public Transportation Association, U.S. Conference of Mayors, International Brotherhood of Teamsters, the Sierra Club, and more than 40 other groups called for the funding in a proposed $3.5 trillion spending bill Congress plans to take up next month. (Reporting by David Shepardson) Our Standards: The Thomson Reuters Trust Principles.