Comment by shagie
From the data at the end of https://climate.mit.edu/explainers/freight-transportation
+----------+------------------------+---------------+----------+----------+
| Mode | Freight (bn tonne-km) | CO2 (Mt) | CO2/Frgt | Vs. Sea |
+----------+------------------------+---------------+----------+----------+
| Air | 303 | 155 | 511 | 79 |
| Rail | 10,842 | 170 | 15 | 2 |
| Road | 26,807 | 2,230 | 83 | 12 |
| Sea | 101,486 | 657 | 6 | 1 |
+----------+------------------------+---------------+----------+----------+
Bulk rates and flatter surfaces win.
However at some point branching is required, and the bulk rates also add latency.