The short, timber-bodies containers on flat wagons (occasionally in low-sided opens) were pretty universal around the system by the end of the 1950s, before dieselisation. They could be seen laying lineside at many freight depots. Many individually in marshalled trains but there were several dedicated services. Goodness, my Hornby railway even had one. BR in those days also had a substantial road vehicle lorry fleet at most depots, able to haul them to and from their destination.
Elsewhere, in the USA intermodal started before ISO containers with Piggyback (which they still have to a considerable extent); road trailers, with their wheels, on flat cars. Began in the 1950s when most US operations were dieselised, and these were prestige runs. But I recall Trains magazine managed to find one 1950s photograph of a Piggyback with a (I think it was a Nickel Plate 2-8-4) big steam loco heading the train at speed.