While the book is far from precise on this subject, it's probably worth noting the following:
1) Inhuman Speed is described on YS178 as follows, my emphasis: "You are very fast, just past the edge of human capability."
2) I am led to believe based on a wiki entry that "The fastest human footspeed on record is 44.72 km/h (27.79 mph), seen during a 100-meter sprint (average speed between the 60th and the 80th meter) by Usain Bolt."
My parsing of this is that statement #1 is comparing an otherwise typical/average character that has acquired Inhuman Speed to the absolute best possible mundane human capability (as described by statement #2).
So, if I was to put numbers on paper, I'd probably say that a character with Average Athletics (+1) and Inhuman Speed can expect to reach about 30mph -- assuming a short sprint, average success, and no aspects invoked. A more athletic character with Inhuman Speed and with more of an investment in moving as fast as possible (running a race, running from something that wants to eat him) would be noticably faster.
Based on this, I would say that a character with Inhuman Speed might be able to keep up with that 40mph train if he had a significant investment in speed-related stunts, athletics, etc, and rolled well. Maybe. A character with Superhuman Speed ought to be able to catch up to the train fairly easily.
The RAW for Mythic Speed indicates that it allows a character with no other prerequisites to "simply declare that you have no problem keeping up with a moving vehicle". Another vague statement; this could be rules-lawyered to mean that a character with Mythic Speed could casually keep up with, say, a spacecraft -- which is technically a moving vehicle. For the sake of sanity, I'm going to assume that they meant something much more mundane -- say, a car moving at freeway speeds (70mph). And again, I'm going to assume that practically any character with Mythic Speed can achieve this, regardless of other abilities.
So here's a chart to play with, completely abitrarily recorded by me based on the above:
average human (sustained/sprint): 6mph/10mph
optimal human (sustained/sprint): 19mph/28mph
Inhuman Speed* (sustained/sprint): 20mph/30mph
Supernatural Speed* (sustained/sprint): 40mph/60mph
Mythic Speed* (sustained/sprint): 70mph/100mph?
(*) baseline for otherwise average human capabilities
Feel free to disagree!