An instance of weight in relation to speed can be seen in the way that the

GW returned empty vans to the LM. Large numbers of vans had to be returned

via Pontypool Road and Hereford and even though they could have been

returned as Class C or D services, they were actually run as Class E

services. With a Hall 4-6-0 or Black 5 a C and D train could convey 45 and

50 respectively whereas a class E could take 60.

From BR Forum/Divman