You know, I have no idea. We use a mixture of contractors and in house IT engineers. Two of the contractors we use are NexusAlpha (based in Medway I think) and Delta Rail (they seem to be midlands based).
The only thing I can suggest is keep checking your local train companies, Transport for London and Network Rail careers websites. I have seen an IT job advertised on my TOCs website but I don’t know if they’d all do that. There’s no such thing as consistency in the rail industry.