I have assembled a

LibreOffice spreadsheet for your examination and tinkering, which computes the capacity and class of:

- passengers,
- mail,
- or a combination passengers and mail,

given

- the equivalent cargo capacity in tons of a vehicle (or a single hold of a ship), or
- the internal length, in feet, of the passenger compartment

It does this by estimating first the cubic feet of the cargo compartment (based on a standard cubic-feet-per-ton factor for gravel), then estimating the square feet of floor space; then, assuming about 6 square feet per passenger (at lowest comfort level, and accounting for passageways) looks at comfort level (higher comfort equals more space per passenger) and catering (higher catering uses more floor space), to estimate the final passenger capacity and class (1..5).

The numbers for a variety of coaches and railway carriages seem sensible, but the Humber Keel (pondering upon which inspired this spreadsheet) I suggest would have a mail capacity more like 120 rather than the 500 currently. The mail-and-passenger Humber Keel would then be 24 passengers and 40 mail (rather than 20 passengers who somehow now still permit carrying the same 500 mail of the single-purpose vessel!)

I hope this may prove useful. If there is interest, I could code this logic into makeobj, so pakset writers could specify vehicle dimensions, desired comfort and catering, and we would automatically compute the passenger capacity and section class.