Considering the alternative is likely to be starvation for five hundred people and two hundred mounts, I think Mathilde can risk a little miscasting.
After a lot of googling and head-scratching and number crunching, the capacity of the upper deck of each steam-wagon is about 60 cows, which works out to almost exactly one week of food. Which sounds like an ass-pull on my part to keep the numbers simple, so I'll do my primary school maths teacher proud and show my working.
Dwarf/human food requirements: 3 pounds per day times 520ish people equals about 11 000 pounds per week.
Wolf/demigryph food requirements: 25 pounds per day times 200 mounts equals 35 000 pounds per week.
Total: 46 000 pounds per week.
1000 pounds is about right for the cows - steers tend to be bigger than that and bulls much bigger but modern cows have advantages that steppe horde ones don't so it works out, more or less. Figure about a quarter of their weight is bone because these are probably going to be on the skinnier side, so 750 pounds of meat and offal per animal. The Demigryphs and wolves aren't picky, so anything not bone can be considered useful meat.
Steam-wagon capacity is determined through this highly scientific method:
30 per side, 60 per steam-wagon. 60 cows times 750 pounds equals
45 000 pounds.