The British Transport Police (BTP) (Welsh: Heddlu Trafnidiaeth Prydeinig) is a special police force that polices those railways and light-rail systems in Great Britain for which it has entered into an agreement to provide such services. British Transport Police officers do not have any jurisdiction in Northern Ireland, where policing of the railways is the responsibility of the Police Service of Northern Ireland.
The website is [1] and the Wikipedia page is [2]
Access to historical resources here.
The British Transport Police History Group website is here