In many European countries the rail networks have become very busy and tight in schedule. Especially in the United Kingdom, where the network has been privatized and several rail companies make use of the same network. This means that any State Visit using a train, with security before and after, closure of platforms at the station, closure of railway lines for the royal train, etc. cause a major hindrance to the proper workings of the railways.
Back then in the 1950's and 1960's when the railways were not as congested as today, there was more space but also more goodwill to arrange "slots" for the arrivals and departures of royal trains.