Back to question: yes, the main reason was to let those foreigners stranded in Russia give a chance to leave. Plus that the first week it was also used by many Russky wanting to get home suddenly after all flights were withdrawn. For this reason FInland even stopped the previous anti-covid measures: till then these trains were only open for FInnish and RUssian people. Even the need for all those covid-codes etc. were temporarily put aside. I guess the demand for the route has since dwindled enough. And perhaps a fear of Russia suddenly ´confiscating´ foreign stock as anti-measure against all those leased planes they would have to give up. Though I do not know the exact ownership of these trainsets.
Now I also do not think that ´doing it all by rail´ would be the main motive for more as perhaps 2 or 3 dedicated railfans (Germany seems to have more of them as even this UK), those in need would take any means of transportation. But AFAIK there still seems to be a possible way with just a few Kms in between: from Leningrad (or Peter for those believing in modern times) the elektrishka (dirt cheap) to the last stop before the border with EE=Eestti/Estonia, it seems an overnight there would be needed-and this is as its a borderregion only allowed to dedicated real Russians or those with a special permit, then next morning cross that border-assuming this is still open, of which I am not sure. Or if yes-for whom.
The only other remaining country with direct trains is likely to/via Kazakstan, and maybe Mongolia. To China is also all cancelled since covid
Next best would/could be a possible ferry/boat from the Russian coast along Black Sea to north Turkey-there used to be several in normal times.
On the other side-there are thousands, if not more, Russians suddenly stranded-without access to flights (all cancelled) or even to withdraw money by a card, thus effectively penniless when the purses get empty and thrown out of hotels and the like, in many sunny countries.
More or less on this railing subject: just read that the transit trains to Kaliningrad are still running via Lietuva. On the platforms (all fenced off-even more as those for the €* to LONdon) the Lithuanians have set up big posterwalls-at window height-showing pictures from destroyed places in Ukrayna.
(a measure they have possibly also remembered from anti-spy measures in the times of the real existing jolly workers socialist paradises).