Carstairs railway station

Just south of the station, there is an important triangular junction (Carstairs Junction) where the West Coast Main Line (WCML) divides. The north-westerly route goes via Motherwell to Glasgow and the north-easterly route goes towards Edinburgh, where the East Coast Main Line begins. The southbound route goes towards Carlisle and London Euston. The line between Edinburgh and Glasgow is the only part of the West Coast Main Line used by London North Eastern Railway services. Carstairs is also a marshalling point and the final boarding point (both sleeping car and overnight coach) in Scotland for the Lowland Caledonian Sleeper trains from Glasgow and Edinburgh to London Euston.

Northbound (Down) WCML services usually pass the station on an avoiding line (known as the Down Main), away from the platform line (known as the Down platform), but northbound services coming off the chord from Edinburgh (LNER and CrossCountry) usually pass Platform 1: they cannot be signalled from Platform 2, . However, all southbound (Up) services pass through Platform 2 (on the Up Main), they can also be diverted through the down platform(1). The Up Main and Down Platform lines are both signalled for bi-directional working, and are often used as passing loops for passenger and freight services. For example, the early morning departure for Glasgow Central from North Berwick will wait at the Down Platform as a fast TransPennine Express service from Manchester passes.

There is also the Down Passenger Loop (which is adjacent to the station) and the Up Passenger Loop (immediately to the north of the station) which are both used to stop freight services while faster passenger services pass. It is also common for northbound freights to be stopped in both the Down Platform line and Down Passenger Loop and for fast passenger services to be passed between them on the Down Main.

Down platform and old station building in June 1983

The route through the station was electrified in the 1974 electrification scheme that covered the West Coast Main Line between Weaver Junction and Glasgow Central. As part of this the station was re-signalled. The critical point was the connection from Edinburgh on a minimum radius curve to provide a connection into the Down platform whilst avoiding the installation of a diamond crossing. The provision of superelevation through the Up platform for 90 mph running required deep ballasting; this required the platform to be raised. The original station buildings were being retained, and continuous railings were provided to prevent passengers accidentally falling down from one level to the other.[2] This height difference has now been removed as the original station buildings were demolished and replaced with a more modern alternative and the entire platform was levelled off. The only remnant of the original station buildings was the integral footbridge, now adapted as a stand-alone structure.

The route to Edinburgh was not part of the 1970s scheme; however, it was included as part of the late 1980s ECML scheme, with electric services starting to use the line in 1989 (before the main East Coast Main Line (ECML) electric services started).[3]