The correct version of these two sentences when combined is:
When we were in Europe on vacation, we usually flew on airplanes: however, sometimes we took the train.
We apply the semi-colon to indicate that they have different ideas, and that they are clauses being joined into one sentence.
In this excerpt from Act III, Scene I of Shakespeare's "Romeo and Juliet", the two central ideas are Romeo blames falling in love with Juliet as the cause of Mercutio’s death and Romeo blames his own cowardice and refusal to fight as the cause of Mercutio's death. Tybalt calls Romeo a villain. Romeo refuses to fight him because he has just married Juliet in secret. In his place, Mercutio draws his sword and fights Tybalt. Mercutio is stabbed by Tybalt when Romeo places himself between them to stop the fight. Mercutio is badly wounded and dies. Romeo blames himself for he has become "effeminate" because of his love for Juliet, and as a consequence of his cowardice, Mercutio died.
