If you really want to know how fast it goes, just take a handheld GPS unit on the ride with you. Assuming that the GPS reciever gets a decient signal, the reciever will tell you max speed. This is the simple solution.
If someone wants to get creative... find out the time interval at which the cameras flash, flashes. This information, along with the distance in which each picture is taken (realitive to the train) will show the speed of the train as it passes the camera.
If this doesnt work, you could also time (with a stopwatch) the time it takes to travel a certain distance. What distance to use I dont know, but it could be done if someone had a some knowlage(sp?) of the building or the track.
Just remember, there are more ways than one to find out the velocity of a rollercoaster, other than by reading a number that is published.