Is calling out the Runaway Train fallacy an Argument of the Beard?

I would say this is both the Slippery Slope fallacy and the Nirvana Fallacy .

Well it's a speed limit, not a "this is the precisely optimum safe speed figure". It's also a compromise of safety and practicality, otherwise zero would be the speed limit. 

Analogue speedometers probably used units of 10s before speed limits were introduced, and you can stick to roughly those without taking your focus from the road too much. So it's historical and functional. 

Within the system of using those increments, 20mph is measurably safer in terms of reduced deaths and serious injuries at low(ish) speeds, and there is very little inconvenience in such a reduction over short distances.

Reducing it further wouldn't result in as large a safety improvement, whilst making driving  increasingly impractical. 

Just asking the question doesn't commit the fallacy anyway, he would need to say something like "on a scale of completely safe to unsafe there is no way to determine at which precise speed it becomes safe for children in the area therefore we should do away with speed limits". 

I don't think you commit logical fallacies by asking a question in that form. It is actually a valid question. Why not go lower? As I understand the Beard argument, if the arguer said "Lower it from 30 to 20? That's not a real reduction", then I think that is a Beard argument.