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Near constant engineering works, signal and track failures.
Plus most tracks are running very close to capacity, so it only takes one small incident to cause major knock on effects as trains get delayed, retimed, etc and the platforms have to shuffle around to accommodate them.
Most of the time trains are consistent, except when they're not.
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At Manchester Piccadilly there are 14 platforms of which only 2 are on through lines.
There are only 4 lines for most of Piccadilly's approach.
At Ardwick, the lines from Stalybridge (for Sheffield) and Glossop merge together then join the main lines on the east (Platform 1) side.
Further south, the 2 South Manchester (aka Airport) lines join on the West side.
Airport to York trains have to cross from the West to the middle,swap ends at Piccadilly, then take Ardwick junction.
Stockport services (inc Macclesfield, Crewe, Knutsford, Bolton) take any platform between 3 and 11.
Platforms 1-10 can hold 3 services using Northern Rail's stock, but Pendolinos can only use the middle platforms.
Here's a live view of what's occupying each track segment, there isn't much leeway for a service to be delayed without it cascading.
In German train stations they tell you the platform of your train on the printed timetables. In the UK we seem to assign platforms to trains on the fly.
Why?