can combine it all with things on the ground and put that into one simple graphic that feeds to the engineer showing him or her how fast he must get on the brakes to stop before reaching some sort of obstacle up ahead and if the engineer does not do that then all of that information through a computer system will take over his train and automatically slow it down or even stop it to avoid a problem out there. that is the positive part of positive train control, anderson. >> do officials have a sense of how many different types of accidents this would prevent? >> there was congressional analysis of this done and it was substantial. it could get rid of all of the train to train collisions and keep us from having switching errors where a switch has been changed and a conductor or engineer doesn't know it and get rid of the problem of work crews get run into by trains and more