When I was much younger, say in about 1960, the idea of getting "the milk train" back from the sticks into London was quite widespread. There were milk trains in those days, but I don't remember anyone who actually been on one.
The "Milk Train" was an old-style general public euphemism for a very early/late service, and/or one that stopped everywhere. Whether it carried any milk was by the by. In the early days of the railway such services would pick up milk churns at country stations, and the name stuck. Furthermore, milk churn traffic was always seen as "passenger rated" traffic (as opposed to the likes of coal), sent from passenger station platforms in vans similar to passenger coaches, and the train carried one or more vehicles with passenger accommodation, and were generally in the passenger timetable. Even when milk traffic moved on to bulk milk tankers (often 6-wheeled) being sent up to London, these were designed to be run as passenger vehicles. One or two were even attached to the early dmus on Westcountry branches.
In the opposite direction would go the "Newspaper Train", an equivalent operation, and I knew plenty who returned from London (wheer it left at about 2 am) to Taunton. Unlike the milk train, the Newspapers would be up there with daytime expresses for speed, and sometimes had the fastest timings of the day. Not in the timetable, but they always had one or more passenger vehicles, and if you were pleasant to the guard (a necessary prerequisite) you would be allowed in. Sailors in uniform returning to the Navy base in PLymouth (and as so many GWR men had once been in the Navy themselves the guards knew the score) were regulars on the fastest, which ran about an hour ahead of the one stopping at Taunton, hammering through there in the early 1960s in the middle of the night with a King ruunning at Cornish Riviera speed; flat out to make the charge at Wellington bank. All the heavy freight on that line, which mostly was run at night, was kept well out of the way for The Newspapers.