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The train I got this morning[1] was showing that. On Tuesday my trains are planning about half of the usual schedule, the same timetable as today, and all showing that. I don't know which train you are planning to get but I'd trust if for Tuesday evening.
[1] GreatNorthern rather than Thameslink but it's all GTR and basically the same
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Real Time Trains shows them as STP (short term plan) which means the train company has uploaded something instead of the usual timetable, so it should be what they’re intending to run.
The amended timetable for Tuesday is live on https://www.nationalrail.co.uk/ so whatever that says should run.
(Source https://www.thameslinkrailway.com/travel-information/plan-your-journey/~/link.aspx?_id=6B114E5EF81746A39DBBF9EFC4CE2CAD&_z=z )