Teacher has got it working. The problems were: The wheels weren't pushed in enough and the wheels needed to be filed down a little. It works now, but is not really strong. It's quite a small motor for such a big train.
We tried out our solar train again today (now that teacher has filed the wheels). It worked so well. It worked best with just the engine, on tarmac, then concrete. It worked ok with a carriage, but was a bit slower when we put on a carriage and the caboose.