Old cast-iron radiators came in two basic types: 1) Those that could be used for either hot-water heating or steam of all types, and 2) Those that could be used only on single-pipe steam systems.
This is a video in infrared of a single-pipe type radiator heating up.
A single-pipe radiator's individual sections are only connected at the bottom. It works, as is clear in the video, by the steam entering the radiator from one end and rising up into the sections one at a time and displacing the air in one section at a time and propelling the air out of the vent at the end opposite the inlet valve.
In general, a two-pipe steam/hot water radiator heats up differently wherein the hot fluid, be it steam or hot water, rises up in the radiator and flows along the top connections. There are other videos on YouTube that show this, but this is one of the few (if not he only one) showing a one-pipe steam radiator heating up in its idiosyncratic way.
Because the boiler heating this radiator has a low-water cut off that, by design, shuts off the boiler every ten minutes for a period of one and a half minutes, the steam filling the radiator is noticeably delayed.