I could see either method as being valid...
Gold leaf can be obtained in roll form & in preset widths. I've never used it, so can't vouch for how clean the edges are. Normal gold leaves have a very slightly ragged edge, so I'd expect roll leaf to be the same.
A broad red line followed by gilding would probably be quickest to apply, with the challenge of then running a line of gold size exactly up the middle, but gilding followed by pinstriping would tidy up the edges of the gilding. I believe Midland lining was nominally 1/2", so that might suggest the second method if they were being tight on leaf. Swindon used a nominal 3/8" line, so *may* have used 1/2" leaf & scrapped the outer 1/16" edges (collected & cashed in at a goldsmiths?).
Sight of an original section of Midland lining would settle things in about 30 seconds.
Fairly highly confident, as the methods of application are well established, & the basic materials haven't changed much. The formulation of gold size may have been altered, & I'd expect the paints to have a 'modern' synthetic alkyd base, but that's about it - the rest of the materials are very traditional. For instance, it's still quite possible to buy brushes made with bird quill ferrules.
I dare say it's possible to line a moulded coach using modern plastic masking tape instead of trusting to muscle memory, but it'd take forever & look rubbish around the corners. Railcar 22 was a "tape job", but that was because it was sprayed in 2-pack car enamel. It was that experience that pushed me to learning how to do it "properly".
The one thing that *is* different is speed. I'd have got my cards in the Factory for being way too slow, proper time-served people are quick because time=money. for a modern-day example there's this gentleman:—
Blank to fully lined in a little over three minutes, one brush & one guideline.
Pete S.