Originally under BR no freight train was allowed to run that did not fit into every loop on the route. The length of the loops was a factor of the maximum load that could be hauled over the route as shown in the freight train loads book.
Loops were originally designed to accommodate two locos to allow for a failed train to be dragged into the loop, plus an allowance at the entrance end to accommodate assistance from the rear and make sure that the assisting locos could stand in clear.
In the early 1980s the expansion of contained traffic resulted in traffic between Glasgow and Felixstowe exceeding the maximum that could be accommodated as the number of wagons from Basford Hall where additional freightliner wagon sets from Garston and Trafford Park were attached. The resultant train was both over-weight for a single class 86 as well as being over-length for the capacity of the loops south of Crewe.
Rather than run two trains the logical and most economical solution was taken to extend the length of the train. this being 4E76 19:30 Coatbridge FLT to Felixstowe MCT. This took 2 x 86s from Coatbridge to enable it to climb Beattock and Shap. Normally these would have come off at Basford Hall however they continued to Willesden where as far as I remember the train was worked forward by 2 x 37.
Because of the length and the weight, 4E76 was subject to special regulation in the former London Division as the only locations where it could be looped were Rugby on the Up TV slow, Bletchley in the UGL and Kings Langley Up Loop. The supervisors at Rugby and Bletchley PSBs would regularly converse with one-another with regards to the running of 4E76 when it was approaching from Nuneaton and when it would pass Rugby. Wherever possible the booked routing of 4E76 via the old line was adhered to even at the start of the morning peak to avoid the torturous diversion down into what was known as "the black hole" e.g. Northampton and the notorious 15mph PSR at the south end of the station at the bottom of the climb up to the birdcage at Roade.
Care also had to be taken in how 4E76 was run alongside other trains as if it was slowed or stopped, the two 86s would bring out the breakers if power was applied too quickly. This meant that re-starting the train or regaining speed could take a long distance. Many, many times when it was coming off a slack for example or getting a signal check, the ECR at Willesden would ring to report an OCB operation for which the ECR operator would simply simply reset the breakers and restore the traction current when we told him that 4E76 was in section. As far as I know this was the only train on BR that such arrangements applied to it.
Under Privatisation, as freight started to increase, the number of trains longer than loops along the route could accommodate increased. As I think another poster has alluded to the cost of even a small extension to a loop can result in very significant infrastructure costs, especially in terms of signalling especially should entrance signals need to be moved backwards as this will affect the spacing of signals to the rear potentially for some distance. That is why loops have not generally been extended.
I think long trains with limited looping capacity will be a long-standing feature of modern operations.
Hope this helps.
