As per the details within
http://www.rmweb.co.uk/community/index.php?/topic/53334-royal-scot-express-in-the-1950s/ the Royal Scot trains where no longer than 13 carriages and those 13 MK1 carriages where a lot smaller than the carriages of the IEP trains.
5 IEP carriages can take the same number and more in the number of passengers than would have been carried within the 13 Royal Scot carriages.
"A lot smaller" is vague. To be precise, a mark 1 carriage is 20 metres long but it depends what measurement is taken. The measurement across the body ie outer corners, and headstocks was given as 63 ft 5 inches. The outer ends of the gangwayed stock were bowed, giving 64 ft 6 inches extreme length, and then there are the gangways in addition. Say 20 metres in round figures.
I travelled on the 10 am Royal Scot several times a year all the way through the 1950s. For the record, the third class (later second class) return fare London to Glasgow was just over £5 and a four course lunch cooked on the train was 8/6, ie 42.25p. Early in the 1950s, reduced price mid-week tickets were introduced for travel on Tuesdays, Wednesdays and Thursdays. There were open tickets ie travel any time Tue, Wed, Thu, and a simple form of yield management.
Like most trains of its time, the Royal Scot was not a fixed formation train. There was a fixed part and a variable part. The fixed part included restuarant cars in both classes, kitchen facilities, a brake first (BFK )and a brake third (BTK), and a mixture of corridor firsts (FK, 42 seats), corridor seconds (TK, 48 seats), and open seconds (TO, 64 seats). At busy periods additional coaches were added, usually mark 1 TOs but often older vehicles. I once counted 17 at Euston in the summer of 1955; it was the first complete train to be put into the then-new maroon livery. The RS tended to be fully booked during the summer high season. At other times booking was not normally necessary. There was a relief stopper which left London and Glasgow at 10.05.
The train was commonly hauled by one or other of the two diesel locomotive pairs, which would normally lead to punctual running. If a Duchess class was in charge, running could be erratic.
The point of the story is that
(1) there was spare rolling stock available, including older vehicles
(2) a policy of operating flexible formations avoided dragging empty carriages around the country,
(3) platforms were long enough to take trains 360 metres long and
(4) third class passengers could enjoy more space and comfort than standard passengers get today.
For the record, the schedule was 7 hours 15 (summer, non-stop) and 7 h 40m (winter, stopping at Crewe and Carlisle).
Taking an inflation rate of about 50 x, standard fares now seem to be higher than they were then, but today's advance fares are somewhat cheaper than the reduced price mid-week fares of the 1950s.
Conclusion: present day operators could usefully learn from some features of practices 60 years ago.
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It also depends on how thick the seats are, as I have more leg room in a Voyager than a Mark 4 as such the former is more comfortable even though it has less padding. As such there is space for the thinner seats to be located closer together and still achieve the same amount of passenger space (or in the case of the Voyagers more space).
The same is happening on planes, EasyJet have new thinner seat which give their passengers more leg room (although that is a side effect as they are doing it due to the seats being much lighter) and I am unaware of mass objection to that (although there has been some complaints, although more to do with the smaller tables sizes than the concert level).
It also depends on the angle of the seat back. I agree with you that thickness of the seat back does not necessarily add to comfort. With the 1.8 metre bays on Electrostars, facing passengers have to avoid each others feet, and thinner seats would not make much difference. 1.9 metres reduces the problem. Trains like the Danish IC3 and Swedish X31 appear to have 2 metre bays (and people stlll complain about that stock!). The centre cars of IC3 trains have a stated length of 16 metres and 8 bays. I do not travel around with a tape measure.