The Chariot is a card symbolizing the power of the mind. Control over emotion must be achieved before we can see logic and reason. This entire story is a trick of the senses. The opulence of the trains interior is whisked away to reveal the sterility of the lab. The Mummys wrappings mask a desperate soldier. The voice of the computer hides the identity of the one controlling it. The Doctor cannot care too much, at least not in the moment. Otherwise he cannot think clearly. As he stated in an earlier episode, Clara is there to care for him, to represent his heart, so he doesnt have to give in to emotion. Clara in turn is struggling with reconciling her feelings of guilt, horror and sadness with the wonder, joy and excitement she experiences at the same time. She is coming to realize that traveling with the Doctor is all of these things at once and learning to control her anger and frustration.