I guess old physics teachers just can't quit! 
The example of the convertible and baseball is accurate. To be precise, I should say a 90 mph baseball relative to the ground.
The Mythbusters took on the problem of whether the damage caused by two autos each travelling at 50 mph toward each other head-on was really the same a single auto hitting a brickwall at 100 mph.
They found that the effect wasn't really quite as much as the single car at 100 mph. But, that was due to the way materials deform, etc. The relative speed was still 100 mph between the two - just didn't produce the damage.
Now - back to trains!
Jim
