COLORADO - For 15 years, RTD has had one layer of safety to prevent train-on- train collisions - the reliance on light-rail operators enforcing train separation by following the red, yellow and green wayside signals installed on much of the system.

Now, Regional Transportation District officials say they are about six months away from having a second level of collision prevention in place.

They are in the final stages of testing and approving an automatic train-stopping system that will cause a light-rail train to automatically apply braking and take away propulsion power if an operator goes through a red signal, said Cal Shankster, head of RTD's rail operations department.

"It's an added layer of protection," he said.

Even so, no rail-safety technology is foolproof, experts say, citing preliminary findings from the National Transportation Safety Board on the recent fatal crash on the Red Line of the Washington, D.C., Metro rail system.

The NTSB is investigating whether an automatic train-control system, which should have alerted the following train that another was stopped ahead, might have been faulty. Nine people were killed in the Red Line accident.

RTD ordered an automatic train- stopping system when it planned the southeast corridor train as part of the T-REX, or Transportation Expansion Project, project for Interstates 25 and 225, but design problems prevented the agency from using the technology after southeast trains started running in 2006.