The victims are all described as adult Hispanic men.
One was killed, and two others were airlifted to Morristown Hospital with severe injuries.
The area they were struck has numerous "no trespassing" signs.
They were struck by Train 855 which left Hoboken at 5 p.m. en route for Hackettstown. There were 30 people on board; none were hurt.
Service is currently suspended on the Morris and Essex line in both directions west of Dover. Bus service is replacing train service to Mount Arlington.
There is no effect on service east of Dover.
It's not clear what the men were doing on the rail bridge. That is under investigation by several police departments and the Morris County Prosecutors Office.