WASHINGTON, Feb 1 (Reuters) - Major U.S. transit unions and a group representing public transportation systems on Monday asked congressional leaders for nearly $40 billion in new government assistance -- twice as much as President Joe Biden has proposed. In a letter, the American Public Transportation Association and unions asked for $39.3 billion in emergency funding, while Biden has called for $20 billion as mass transit demand has plummeted in the face of the COVID-19 pandemic. Congress has approved $39 billion since March to aid transit systems, including $14 billion in December.