Can US DOT regulate train crews and working hours apart from rules enacted by law or specified in contracts?

US rail workers demanded paid time off and better working conditions, but the Biden administration and Congress prevented them from striking, and though they received raises, their most pressing concerns were not addressed in their latest contract.

Could the executive branch (i.e., US Dept. of Transportation) issue regulations on its own authority limiting work hours and specifying minimum crew numbers, independently of specific legislation to that effect and regardless of what the current labor contract says?

Limiting work hours would reduce the need for time off, and increasing crew levels would improve safety, reduce on-the-job stress, and expand the pool of available substitutes if workers need to take time off.