What we now know as the Northern Line was born out of the amalgamation of the City & South London Railway (C&SLR) and the Charing Cross, Euston & Hampstead Railway (CCE&HR). In 1903, the two companies were granted permission to build underground railways passing in the area near Euston. The owners of Euston, the London & North Western Railway, stipulated that each underground railway must have a separate station buildings. The C&SLR built theirs to the east of Euston, on Seymour Street (now demolished); the CCE&HR built theirs to the west, on Melton Street (scheduled for demolition as part of HS2 works).
The C&SLR and CCE&HR agreed to build a tunnel to connect their platforms, to save passengers from going above ground if changing lines. However, as the lines were run by different companies, a new ticket was required when changing – so a ticket office was installed part way along the linking tunnel (reputedly the only example of this on the entire underground network); there was also a joint ticket hall under the mainline station with direct routes to each line. When the underground lines were unified in 1914, the individual station buildings closed, leaving the ticket hall under the mainline station as the only entrance and exit.
By the 1960s, the lines had long since been operating through Euston as the Northern Line – though as separate branches (still a source of confusion to unwary travellers today). It was the construction of the Victoria Line that finally closed the linking tunnel between the two branches in 1962, with new tunnels and ticket hall being built to cope with increased passenger numbers.
The London Transport Museum operates tours of the disused CCE&HR station and linking tunnel, along with a walk into the ventilation system above the Victoria line platforms. Though converted for service use, the old tunnels were never stripped or cleaned before the modern equipment was put in place; as such, they are something of a time capsule, with walls covered in advertisements from the 1960s, most still remarkably readable over 50 years later.