On a single line, a train doesn't have to fit in a loop to be passed by a train in the opposite direction, providing that only one of the two trains is longer.
Looking back to around 25 - 30 years ago, so my memory may not be entirely correct; but Automotive car-carrying trains were 100 s.l.u. or slightly more (if 10 Cartics + 1 Autic). I have a distinct recollection that one of the services (to Southampton?) was routed Coventry - Leamington and would use Kenilworth loop to pass trains in the opposite direction. They were too long for the loop, but this was not normally a problem because the car train would come to a stand in the loop, the other, shorter, would train draw up to the starter - obviously, with the tail of the car train in front, turning on to the loop. The tail of this train would be clear of the exit points for the car train, and, he line forward having then been cleared, the car train could then move off, and when its tail had cleared the entry points, points changed and starter cleared for the other train to set off, too.
Both loaded and return empties services followed this route, and the process remains clear in my memory because of the chaos that was caused when the loaded service passed Coventry and was sent forward at about the same time as the (very late running) returning empty service presented itself at Leamington and was, without thinking things through, also sent forward. We tried to keep a low profile while Railtrack worked out a solution - I never did know what it was, but it couldn't have been easy.
As an aside, the Harwich P.Q. trains were of similar length, and exceeded the lengths of most loops between there and Stratford, so they always had a clear run!