I haven't been on the MRT for a month and a half, but I may need to use it later this week.

But remembering how rush hour used to be, I wonder if they can require any real social distancing on the trains and in the stations.  Can anyone tell me how successful they are at managing it, if at all?

Nope.  It worked OK while most of the city was in partial lockdown, (every other seat marked with an 'X') but now things have returned to (the new) normal, it's not practical in the rush-hour.  

I was on a bus, today.  All good, but thought how is it going to work out with more going back to work, let alone mobs of students.  Would need much more capacity on the system.  I have been avoiding MRT, but at least I can now go to Silom the opposite way, with the same number of stations.  Was avoiding Sukhumvit rush hours like the plague before the plague.