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.