Hi Frank,
The Sperry wagon is an ultrasonic testing machine which scans the rails to monitor the integrity of the steel.
The tank wagons are only in the consist to provide extra braking power, as the Sperry is apparently quite heavy and the loco brake on 071s is 'soft'. It's really just a case of attaching whatever wagons are available at the time. The Sperry train has previously used the new spoil wagons for the same purpose: http://www.flickr.com/photos/be216cd1/6754449143/