Considering that the distance of the Crimean region to St. Petersburg is roughly about 1,382 miles, and these flowers were brough in daily, I'd say that the Imperial railways had to have had a special engine that would carry the flowers (as well as other items on request by the imperial family) to the capital.
Lilacs bloom in the summer time, the regional temperature in the Crimea during the summer months is a delicious 60-70 degrees F.
I doubt that she would be able to order them any other time of the year except late spring-late summer...but I could be wrong.
As for their freshness...the train journey from the Crimea to the imperial capital would be a couple of days, but lilacs, like several species of flowers can still have a fresh odor and texture to them. For instance, lilac candles can be purchased, used and stored, but can still smell as fresh as the day they were bought.
I'd also have to suggest that because of the war, and because of Russia's large, but slim railway system, every car, engine, and yardworker would need to be on hand for the war effort. Thus, declining the Empresses chances of obtaining her lilacs during the war.
