The wikipedia article on intermodal transport suggests close to 70% of intermodal goes on railroads in the US[0]. I am not sure what the breakdown is in terms of origin (ship vs truck) before it gets on the railway, but it seems reasonable that the ship -> train is at least comparable to, if not larger than ship -> truck. But I imagine it depends on cargo and its final destination. If it's more than a day's drive and is a larger shipment (more than 1 container), I suspect that makes it more likely for a train to get involved.

The same wiki article later says trucks are often used to go between ships and rail[1]; depending on what fraction of shipments experience that, it might be an even stronger argument for rail the whole way, if it saves 2x loading/unloading from trucks.

[0] https://en.wikipedia.org/wiki/Intermodal_freight_transport#C...

[1] https://en.wikipedia.org/wiki/Intermodal_freight_transport#T...