According to the article Russian and Ukrainian subways serve two purposes.
Public transportation and bomb shelters. That's actually smart!
It also demonstrates that Russia cares for their citizens.
The live drill with 40 million people was that so the people knew the locations of all of their subway bomb shelters.
In that article is a video from Ukraine which shows them testing a large blast door closing in a subway system. It is impressive to watch. All 58 seconds.
Begs the question?
Why has not the USA done the same with its citizens.
Pretty awesome, I know. Ever since WW1 and WW2, subways have been used as public bomb shelters, simply due to how built up most of them are.