Which begs another question. Why would a new fleet addition, as a unit marked as a new concept, not be fit for service and require several months of work by the TOC to make it fit for service? Whoever agreed to take on these and not ensure that Porterbrook had them fit for service should have some serious questions to answer.
I can't explain all the details but can only go on what has been posted in this and other threads and Forums and Groups. Certain mods have to be carried out to make TFW friendly, a continuous round of investigations and problem serving as faults showed themselves along the way during testing. . Having to put the later batch which came by road together and all this would increase Depot workload. I can only assume that some were put to one side while work continued on others to try and get some into a fully working condition. On the point of Porterbrook having them fit for service, how many classes create problems after leaving the manufacturer