I agree with Tony's track circuit explanation.
When working in freight yards it wasn't so important but the 03's were often used as station pilots on the Eastern Region where 100% operation of track circuits was essential for obvious reasons.
Here is a picture of an 03 on station pilot duties at Newcastle. It is attached to a similar flat wagon.
http://www.johnwhitby.fotopic.net/p23050858.html
08's and 09's are also used as station pilots but they are much heavier and have a longer wheelbase than an 03.