The Penwortham Triangle was situated on what had been the West Lancashire Railway (WLR) Company’s Southport & Preston Railway which had opened in stages between 19 February 1878 and 6 September 1882. It was located on the south side of the River Ribble to the south-east of the settlement of Penwortham.

When the WLR opened its final section of the Southport & Preston Railway from Longton to Preston in September 1882 there was not a triangle at Penwortham, only the route of the line running up to the WLR Ribble Bridge that carried the line over to the north side, and into Preston where the WLR had their own station. Just seven months later, on 16 April 1883, a line opened from what became Penwortham Junction to the Lancashire & Yorkshire Railway’s (LYR) Preston and Blackburn Railway at Whitehouse, a distance of 79 chains. The line allowed through running from Southport to Blackburn. So that trains could run directly from WLR Preston station to Blackburn a line was also installed that diverged from the original WLR 26 chains north of Penwortham Junction, at what became Ribble Junction, and from here it ran north-to-east for 22 chains until it joined the line to Whitehouse at what became Middleforth Junction. This arrangement created the Penwortham Triangle. All of the lines were double-track.

Penwortham Junction was controlled by a McKenzie & Holland signal box east of the tracks, which opened with the line in 1882 before the junction was is use. The box had a brick base with a wooden top and contained 16 levers.

At Ribble Junction there was also a McKenzie & Holland box with 18 levers. It had a brick base and back-wall and timber superstructure. The box was located just south of the Ribble Bridge, south of Leyland Road on the west side of the line.

At the eastern side of the triangle Middleforth Junction was controlled by another McKenzie & Holland box that opened with the line in 1883. Passenger and goods services were operated by the WLR over all sides of the triangle.

On 1 July 1897 the WLR was amalgamated with the LYR. The LYR upgraded the former WLR route and their plans brought about changes. The LYR had their own station at Preston (opened in 1838 and shared with the London & North Western Railway) and they had no need of the WLR facility. They decided to close it and run Southport services into the joint station. To do so they needed to build a west-to-north spur at Whitehouse, for which an Act was obtained on 1 July 1898. The contract for its construction was awarded on 26 October 1898 to J Strachan for £9,713. In the same year a new 16-lever LYR frame was installed at Penwortham Junction. A new siding was installed on the down (Preston direction) side of the line, south of the signal box.

On 16 July 1900 the new line opened from Whitehouse West Junction to Whitehouse North Junction which created the Whitehouse Triangle, only 30 chains east of Middleforth Junction.

With the opening of the new line Preston WLR station closed. Middleforth Junction box closed around this time and by 1905 the junction itself had been removed, although track remained in situ from just beyond the site of the junction up to Ribble Junction. The Middleforth signal box structure was not removed until 1922. From 1905 there was no longer an operational triangle at Penwortham.

Ribble Junction box closed around 1902 and was dismantled in 1922; the north-to-east spur would have been removed by this time. The original WLR line into Preston WLR station, which remained open for goods services, effectively became a long siding from Penwortham Junction controlled by Penwortham Junction box.

In January 1922 the lines became part of the London & North Western Railway and a year later passed to the London Midland & Scottish Railway (LMS). In 1948 the former WLR lines became part of British Railways London Midland Region.

In December 1960 Penwortham Junction signal box burned down. It was replaced with a ground frame of two levers, opened on 18 June 1961, electronically released from Whitehouse West box. At this time the line to Preston WLR station was singled. On 7 September 1964 the former WLR Southport & Preston Railway closed as a through route. The line between Penwortham Junction and Hesketh Park closed completely. So that a daily goods service could operate to the Preston WLR station the ‘up’ line from Whitehouse West Junction to 300 yd south of Penwortham Junction was retained for operational use; the down line remained in situ but out of use. To the east of Whitehouse West Junction the line remained double-track. On 25 January 1965 the lines from Whitehouse South Junction to the former Preston WLR station closed.