The Railracing Angels. The Guardian Angels, #3

Danny and Lizzie have passed their individual tests, and now it's time for their pair test.  Helped by their guides, Tom and Barney, who've also died following crashes, they set out to stop two teenage boys from doing a dare to race against a tram, with one of them sticking to the front of it with suckers and the other racing it, to become part of a gang. With persuasion and practice of their new transportation skills, they make the teens see for themselves what they are doing is wrong.
But just as Danny and Lizzie think they are winning their test, a new enemy of a huge thing with dark and dirty wings appears to be trying to get the teens to carry on with their challenge and ignore the angels.  Thankfully, Danny and Lizzie win the challenge, and back at the School they are shown how they have helped the boys to get on with their lives.  And Jerry, the school founder, tells them that he thinks the invisible force was probably an old friend of his who has died, but turned bad and into a bad angel, called a 'Black Angel'.  Now Danny and Lizzie have to train harder, and will have to be on alert more.