The railway is private property so you don’t have an intrinsic right to be there. You have an implied right of entry but the legislation dealing with railway trespass makes it clear that, to all intents and purposes, if an authorised person (ie a staff member) asks you to leave, and you refuse to do so, you are likely to be committing the criminal offence of railway trespass. Holding a ticket is neither here nor there.
CPS guidance below gives a good overview:
www.cps.gov.uk
(It’s a table so difficult to quote from, but Railway Regulation Act 1840 section 16 looks relevant).
IIRC There’s another bylaw offence around failing to obey instructions of staff.
This is why it’s worth speaking to staff first if you want to go and take photos, watch trains or do anything they might make your behaviour seem different to a regular passenger. This is because of obvious concerns around security and people who might be a suicide risk.
EDIT: I’m not commenting on Euston specifically, or in any way defending the staff, because there are clearly
major issues there! At my London terminal platform staff will generally want to see an empty (of regular passengers) platform if trains are going to be splitting and joining. Enthusiasts who want to take photos are certainly allowed to do so, so long as they make it clear what they’re up to.