It may come as a surprise, but there is spare capacity on the route into London Euston, all the way out to Queen's Park.
These are the so-called New Lines opened in 1912, which comprise a two-track suburban railway running from Euston to Watford, and now run by London Overground. At Queen's Park, trains from Euston are joined by Bakerloo Line trains which run northwards as far as Harrow and Wealdstone.
Between Euston and Queen's Park the service is lightly used, there being little traffic at the two stations, Kilburn High Road and the almost-deserted South Hampstead.
If the New Lines could be used by some of the outer suburban trains that run towards Milton Keynes and Northampton, this would release capacity on the four tracks of the main line. It would also be possible to extend the Bakerloo Line the 4km from Queen's Park to Willesden Junction.
How the route might be used
The details of such an arrangement would of course need working out. The lines follow a complex arrangement of burrowing junctions at Chalk Farm, which provide flexibility in routing, before entering the twin-bore tunnels at South Hampstead on the east side of the running lines. At Queen's Park, the northbound and southbound lines diverge, and the Bakerloo Line reaches the surface between them. Queen's Park station has island platforms between the Bakerloo and Overground tracks, and the remains of platforms on the four main line tracks.
The Bakerloo Line tracks continue parallel for a short distance, passing through car sheds before merging with the Overground tracks, which then continue on the east side of the main line until Willesden Junction, where there is a bay for terminating trains. The next stations northward are Harlesden and Stonebridge Park, and the line then dives under the main line tracks, continuing northwards on the west side of the main line tracks from Wembley Park to Carpenders Park. The New Lines then separate from the main line, calling at Watford High Street before reaching the terminus at Watford Junction.
The main difficulty that would need to be overcome is that the main lines are arranged (from west to east) down fast, up fast, down slow, up slow, which means that a grade-separated junction would be needed to enable trains on the slow lines to cross onto the Overground tracks without conflicting movements.
However, if only the stretch of line south of Willesden Junction, were taken over, which is adjacent to the slow lines, the difficulty of conflicting movements is much reduced. But that raises the question of what route a main line could follow from Willesden northwards.