https://www.bbc.com/news/world-europe-61583492
"Ukrainian farmers have 20 million tonnes of grain they cannot get to international markets"
Let's say you could ship 10 ton (10,000 kg) per train car; that's still 2 million rail cars. The longest possible freight train would be 100 cars or so - so now we're down to 20,000 grain train runs needed for the export.
It's not possible, not enough rail capacity, not enough rail cars and engines or days in a year to do it.