Well they kinda don't have speedometer in the older consists, so yeah - speed is guesstimated.
There's a way to estimate how fast the train was going (fairly accurate average speed/velocity, not the instantaneous speed/velocity) if you were at Hoboken Station platform (Tunnel End), and if you timed this (in seconds) how long it took all cars to go through from beginning of the first car and to the end of the last car of the consist that crashed bumper block.
Speed Conversion is as follows:
Length of Unit (PATH runs 7-cars trains, in this case): Approx. 350 feet
Divided by time (seconds): Let's say 10 seconds (if someone happened to witnessed Hoboken Train Crash, please verify this.)
Multiplied by 60 x 60: (3600)
Divided by (5,280) feet per mile
35 x 3600=126000
Therefore, then if it took 10 seonds for the entire 7-car trains to enter Hoboken Station platform and clear (when hitting the bumper?), then it is running at around 23.86 mph. (Of course, this is just an example. But this is a useful way to estimate how fast the trains are going when they fly by you.) The faster the trains clears, the faster the train is running.