When I was looking for a week placement in train planning (back in 2019), I discovered that it's very difficult! However, if you can find the right person (or someone who knows the right person...) it generally goes quite smoothly.
For example, one of the operators I emailed replied with this:
Unfortunately due to the nature of the business there is no formal process for work experience, however on occasions in the past something has been arranged directly with the local station / depot. If you contact the Station Manager / Staff Manager directly they may be able to arrange something for you or provide you with further advice, you can do this by writing to them or by visiting the station / depot in person.
I posted a similar thread to you at the time, and a forum member very kindly put in a word or two in the right places. A week or two later, I got a reply to the above email from the head of train planning saying yes!
I would recommend emailing all your local operators (and Network Rail?) and ask them if they can offer anything; if you can find a general careers email address, that's probably a good place to start. If that doesn't get you anywhere, ask in person at a local staffed station (or maybe try several).
(caveat: I do not work in the rail industry, so I may be talking complete rubbish!
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