Hello, thanks a lot for your replies. Yes, Sissa = Scuola Internazionale Superiore di Studi Avanzati. I think it had windows all along, but I am not completely certain. I only saw the car once, although I walked past there every day for a month. The railway track I saw passes underneath Via Boromeo, and in fact the bus stop nearby is called "via Bonomea 184 (ponte ferroviario)", because the road goes over the tracks there. I think the coordinates of the railway bridge is approximately 45°40'40.0"N 13°46'24.3"E. I tried to trace the tracklines on google maps in either direction , which is how came to my initial conclusion that it somehow connected CM to Opicina.Hi and welcome to the forum. I am no expert on Italy but:
I presume Sissa means the International School for Advanced Studies SISSA? If so, the tramway follows the main road quite closely and crosses your access road Via Boromeo, so that's ruled out.
You can certainly see the historic Transalpina railway down to Campo Marzio. A quick Google suggests the line has been reopened for freight traffic, but not for passengers; there is also a railway museum at CM.
I wonder if you saw a single locomotive running down or up the steep gradient? Or a railcar (did it have windows all along? If, so, yes it was!) - perhaps to take staff to and from CM, or for driver training? Or possibly an historic car )from the railway museum, but that seems less likely.

(railway bridge behind sissa)
Perhaps it is a tram. However, the Youtube video I saw of the tram route (as recommended above) seems to pass through completely different locales (closer to via Commerciale) so I'm a bit confused.