Just south of the station, there is an important triangular junction where the West Coast Main Line 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 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 in Scotland for the Lowland Caledonian Sleeper trains from Glasgow and Edinburgh to London Euston. Northbound WCML services usually pass the station on an avoiding line, away from the platform line, but northbound services coming off the chord from Edinburgh usually pass Platform 1: they can be signalled from Platform 2, but this rarely happens. However, all southbound services must pass Platform 2, as there is no avoiding line on that side of the station. 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 and the Up Passenger Loop 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.
Electrification
The route through the station was electrified in 1974 electrification scheme that covered the West Coast Main Line between Weaver Junction and. 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 requirement for superelevation through the Up platform for 90 mph running required deep ballasting the side effect of which required the platform to be raised. The original station buildings were being retained therefore continuous railings were provided to prevent passengers inadvertently falling down from one level to the other. 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 to remain 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.
Services
Historical
Carstairs was an important junction station where northbound West Coast Main Line trains were split into separate portions for Glasgow, Edinburgh and Stirling and Perth, and for the corresponding combining of southbound trains. However, the introduction of push-pull operation on the WCML and the availability of surplus HST sets for Cross Country traffic largely eliminated this practice in the early 1990s. Apart from the sleeping car trains, express traffic through Carstairs now consists of fixed-formation trains which do not require to be remarshalled en route. As a result, few express trains now call at Carstairs. There were some local stopping services to Edinburgh and Glasgow, but they were relatively infrequent, Before December 2012, only two trains per day to North Berwick called and only five trains to Glasgow There were very large gaps in between trains with the two Edinburgh-bound trains calling at 07:49 and then again at 15:40 as with the Glasgow trains a nine-hour gap from 07:55-18:41.
2019
The following service calls at Carstairs, On Monday to Saturdays: There is a roughly 2 hourly service to both Glasgow Central and Edinburgh Waverley for most of the day with a few services extended to Ayr and North Berwick, the last northbound service from Edinburgh terminates at Motherwell, There is also a few extra trains which call at peak times to/from Glasgow Central Low Level operated by ScotRail which operate to Motherwell, Garscadden and Dalmuir therefore a slightly reduced service operates from the station on Saturday mornings, In addition, TransPennine Express also provide one train per day to Glasgow Central and one train per day to Liverpool Lime Street via Preston. As of 2019, there is no Sunday service.
