WASHINGTON (Reuters) – U.S. President Joe Biden is expected to call on Congress to intervene to avert a potential rail strike that could happen as early as Dec. 9, a source briefed on the matter told Reuters. More than 400 groups earlier on Monday called on Congress to intervene in the ongoing railroad labor standoff that threatens to idle shipments of food and fuel and strand rail travelers while inflicting billions of dollars of economic damage. White House press secretary Karine Jean-Pierre declined to confirm Biden’s plans but reiterated that a “shutdown is unacceptable.” (Reporting by Dav…