There is no uniform way to translate the real-world capacity of rolling stock in Minecraft inventory slots. Resource pack makers can at best make an arbitrary estimate based on the size of the container, leading to an imbalance when numbers of slots are not determined in the same way.

In the interest of maintaining balance between different item carriers across resource packs (and item-burning fireboxes), resource pack makers should be prompted to enter the real-world total weight capacity of the item carrier (probably in kg). This value could then be plugged into a formula that calculates the number of inventory slots based on the weight capacity entered.

For example, a boxcar can carry 40000 kg. A formula used could be [capacity] * 0.00125, giving it 50 inventory slots.

Item-burning firebox grates could be calculated from the grate area in square meters. For example, if the formula used is [grate area] * 2, then a Big Boy would have 28 firebox slots and an A1 Peppercorn would have 9, assuming realistic values are used.

The multipliers used could be changeable in a config file, or updated with IR itself if the default values are deemed to be inappropriate.