The title attribution poses another riddle: "PDP-1/B and miniature model railroad at University of Massachusetts".
The PDP-1B was the production prototype of the PDP-1 and there were only 2, we know of. The first one (white) was intended for LLNL, but the order got delayed and it went to BBN instead, the second one (blue) became the actual LLNL machine.
Update: Curiously, the this page [1] also mentions a "PDP-lB" [sic!]:
Good article... it points out that J.A.N. Lee was appointed Program Head of the new CS program at UMassAmherst in September of 1964. And that is the same person whom the 2nd photo was from in the thread’s main article.
Unless things have changed it would be unusual to refer to the University of Massachusetts as UM. I grew up in MA and have only ever heard it called UMass. It's possible it used to be called UM before they expanded it beyond Amherst and Boston.
OO is just a slightly bigger scale (1:76) at H0 track. Since UK loading diameter is slightly narrower than on other standard gauge railways (which is the reason for there being OO at all, as H0 motors wouldn't fit models of UK locomotives), there wouldn't be much of a noticeable difference. To me, this looks bigger than H0, but there are other scales and gauges, as well. Admittedly, my knowledge about these is pretty limited.
It's definitely an American HO scale layout. The track looks like Atlas and I recognize several of the buildings as HO-scale Revell kits that were popular at the time.
"Model railroad" would suggest a [scale] model of some extent railway, otherwise isn't it just railway/railroad without the "model" designation. If it's not modelled on something isn't it properly a toy or miniature railway. /pedantry
Well for one thing grown people play with all sorts of things. Boats are also quite popular, not just real ones but model ones.
Also there were more "group" activities in those days (consider Moose and Elk lodges and other societies, of which only a few really survive any more).
As far as trains specifically: For a long time trains were the definitional high tech artifacts; my dad bought me a train set (I was born in the early 60s) and though I thought it was merely OK, he, being from the 1930s, remembered trains as super exciting.
Plus it's one of those activities with a gajillion degrees of freedom (mechanical, artistic, electrical, mathematical) which encourages groups of people to work together on the pieces that appeal to them.
Edit: Confirmed, kind of. A similar but different picture is figure 9: https://www.semanticscholar.org/paper/A-Historical-Overview-...
It looks like the picture slated for figure 11, that made it to the CHM archives, didn't actually end up in the article.