I whipped up a basic spreadsheet for comparing mixed consists and freight consists with express in terms of weight.
I was curious as to when it would be possible to use an express loco to haul freight, without overloading it in terms of grunt up grades or in terms of reliability. The idea was that there should be scope for hauling really fast freight sometimes, as long as the load is chosen carefully. It wouldn't be worth the extra fuel cost for most freight, but occasionally you might want to move something quickly.
So I made a sheet that allows checking any freight or mixed consist for any of the 8 eras, and seeing exactly how it compares in terms of load with a full (8 car) express consist. It would be easy to modify to compare with a 6 express car consist too, or any other number, but at the moment full express consists are what I'm running so they are the obvious reference point.
The tables in the sheet are labelled as being for 1850-64 and for 1865-1874, but the ratios of car weights are the same throughout the progression of eras. This is a 25 year cycle for each grouping of cars, with a 10 year stagger between groups.
In relative terms, the 1859-64 tables will also work for 1825-39, 1875-89, 1900-14, 1925-39, 1950-64, 1975-89, and 2000 to infinity. The 1865-1874 tables will also work for 1840-1849, 1890-1899, 1915-24, 1940-49, 1965-1974, and 1990-1999.
As a rough guide: if you want to do the classic consist of mostly express but with a reefer of milk at the head of the train, that will work as 6 express + 1 reefer for any era. It will always equal the weight of 8 express cars to within a fraction of a ton. On the other hand, if you want to haul flat cars of uranium or steel, or box cars of goods or clothing, 3 of any of those will also equal the weight of 8 express cars (or as near as makes no difference) so your loco will perform exactly the same hauling them as it would with its normal express consist. Or you can haul 1 pax + 2 hoppers (or tankers) at fractionally less than the weight of 8 express cars.
Which sounds more confusing than it really is. It's pretty simple, really.
A good rule of thumb is that the ratios of weights go like this:
Express (pax, mail, troops): relative weight = 6 (this is just proportions, not absolute values, so bear with me).
Light freight (reefers, stock): relative weight = 12 (ie: 1 reefer = 2 express cars weight).
Medium freight (box, flat, autorack): relative weight = 15 (ie: 2 and a 1/2 express cars).
Heavy freight (hoppers and tankers): relative weight = 20 (ie: 3 and a 1/3 express cars).
Full express consist is relative weight of 8 x 6 = 48. So that's four reefers, or 3 box cars (plus a fraction you can't use), or two of the heavy cars (plus a fraction you can't use) if you want to stay within the limits of express consist weight.