Welwyn Viaduct and the Welwyn Tunnels not being four tracked by the GNR is another bottleneck.
Like the Catford Loop mentioned elsewhere, the Hertford Loop was supposed to solve that but, again like the Catford Loop both lines attracted suburban traffic.
Further north there is the very early example of the GNR's duplicate "Towns Line" and "Fens Line" between Peterborough and Doncaster. The former was more direct, but more steeply graded (Stoke Summit - in supposedly flat Lincolnshire - is the highest point on the entire ECML) whilst the latter was much flatter, crossing the fens to Boston and then via the banks of the River Witham and the Foss Dyke Canal to reach Retford, making use of the Lincoln Gap to negotiate the ridge that the Towns Line surmounts at Stoke Summit. The Fens Line proved ideal for the very long coal trains that were the GNR's bread and butter - their length only limited by the requirement that they not obstruct all three level crossings in Lincoln at the same time.
The Fens Line survives at each end, but most of the route between Lincoln and Spalding is lost. It was largely superseded by the "Joint Line" which took a more direct route via Sleaford but was essentially built for the same purpose - getting freight off the ECML. As originally built it was even better at that than its modern incarnation, as instead of rejoining the ECML at Peterborough it continued to March, from where the goods trains ran down the GER main line, to the mutual advantage of both the GNR (which had access to industrial Yorkshire) and the GER (which had the capacity through the Home Counties that the GNR lacked, and also access to the London Docks)
Maidstone line has a very sharp curve at Otford junc:
Reflecting that the original route ran to Sevenoaks and the Maidstone line was an afterthought. Given sufficient will and money a bypass could have been built to straighten it out, but it was never the LCDR's main line - that ran from London, through Chatham, to Dover (hence the name)