The Department of Transportation needs to do more to hold railroads accountable for the sluggish adoption of positive train control, a group of 16 Democratic senators said in a letter sent to Transportation Secretary Elaine Chao late in December.

The letter was written four days after an Amtrak train derailed in DuPont, Washington, killing three individuals. The two-locomotive, 12-car train was travelling at 78 mph at the time of the derailment, according to a N...