Getting a train from Kiev to Riga is rather wasteful (about 36 hours with a transfer) and for money (the cost of travel will eventually be higher than the cost of the flight) .The simplest option is with a transfer in Moscow, but you can also by another way: Kiev-Minsk-Riga or even Kiev-St. Petersburg-Riga (it's almost two days on the way) .You can get there with two transplants (on average 2, 5 days): Kiev - Vilnius - Daugavpils - Riga, Kiev - Minsk - Daugavpils - Riga, Kiev - Minsk - Molodechno - Riga .This is all solely by train, if the bus is unacceptable .Otherwise it will be more convenient to take a train to Vilnius (it is possible with a transfer in Minsk), and from there by a comfortable bus .