When was the border to mainland China closed after the revolution

Here are some notes about the effects on the through-train services, though I'm not sure when (if?) the border was completely closed.

Following the end of the [Second World War], traffic quickly recovered but through-train services for both freight and passengers were again halted on 14 October 1949, the day prior to the People's Liberation Army's capture of Canton from the Nationalist forces. Until 1951 both passengers and freight had to be transshipped at Lo Wu between separate trains operating on the Chinese and British Sections. In 1951 through-train freight services resumed, but through-train passenger services did not resume again until 1979.

The Hong Kong Annual Report 1955 gives a slightly different description:

[...] since October 1949, when the Central People's Government was established, through passenger services have been suspended. Passengers proceeding to and from China change trains at the frontier. Goods traffic in wagonloads has been passing to and from China without off-loading at the frontier since 1950.