The southern interconnect between the Northampton loop and the direct London–Birmingham main line is at Hanslope Junction, just north of Milton Keynes.

[1] The majority of passenger services on the line are provided by West Midlands Trains using Class 350 electric multiple units.

Avanti West Coast provide a small number of Class 390 Pendolino services to London at the extremes of the day.

As of 2011, line speeds were expected to increase to 90 mph (140 km/h) once signalling improvements are in place north of Northampton up to Rugby.

Many of these are container trains, with some serving the Daventry International Railfreight Terminal (DIRFT), which is between Northampton and Rugby (thus on the loop line).

When the London and Birmingham Railway (L&BR) was constructed in the 1830s, Northampton was by-passed, with the line running on high ground to the west via Kilsby Tunnel.

As Northampton is located in the Nene Valley, 120 feet (37 metres) lower than Blisworth, the closest point the L&BR came, connecting the town would have required gradients significantly steeper than this.

[1] The successor to the L&BR, the London and North Western Railway (LNWR) decided to construct the loop line through Northampton in the 1870s.

It was built as part of a wider scheme to double the capacity of the West Coast Main Line between Bletchley and Rugby, by quadrupling the track; however, routing the additional tracks on a deviation via Northampton had the advantage of giving the town a much better rail service, including a direct service to London, and avoiding the expense of widening Kilsby Tunnel.

The station needed to be expanded as part of the works, this required the almost complete demolition of what remained of the Castle to make way for it.